Analysis by Ronald Brownstein, CNN

(CNN) -- In next week's midterm elections, President Donald Trump is poised to put his stamp on each party's demographic and geographic base of support as surely as he formerly fastened it to one of his hotels.

A CNN analysis of the demography of the most competitive districts in the House of Representatives, almost all of which are now held by Republicans, shows that the outcome in 2018 appears poised to reinforce the divides familiar from Trump's election in 2016.

Democrats' top opportunities to capture Republican-held seats are concentrated in well-educated, higher-income and preponderantly white districts. Most of these seats are centered on economically thriving suburbs around major metropolitan areas where Trump faces widespread resistance among white-collar voters, especially women, on cultural and personal grounds.

With only a few exceptions, Democrats face more uncertain prospects in Republican-held House seats centered on the blue-collar, exurban and rural communities where Trump remains popular, the analysis found. Of the 43 Republican-held seats that CNN considers leaning toward the Democrats or toss-ups, only nine are in districts where the white population exceeds the national average and the share of residents with college degrees lags the national average.

This stark divergence carries several clear implications, both for election night and beyond.

The trench dividing America

Most immediately, it means that Democrats, while still favored by most analysts to win control of the House, are operating with relatively little room for error because they are trying to gain the 23 seats they need predominantly on one part of the playing field: well-educated suburban seats. Even if they do reach a majority next week, its likely the Democratic hold on the House will be precarious and very slim unless they can also capture a respectable number of the small-town and blue-collar seats now considered toss-ups.

The longer-term implication is that this election now seems highly likely to widen the trench between a Democratic Party that increasingly controls the major metropolitan areas largely skeptical of Trump and a GOP whose dominance is barely dented in the rural and exurban areas where he remains strong.

Already, the CNN analysis shows, about two-thirds of House Republicans represent districts where the education level lags the national average and nearly three-fifths hold seats where the median income is lower as well. By contrast, about 53% of Democrats hold seats where the education level and median income exceed the national average. If Democratic gains next month are concentrated mostly in white-collar seats, the Republican caucus will tilt ever further toward lower-education and modest-income seats while the Democrats will bend further toward the opposite -- expanding the distance between the two sides and making compromise between them even more difficult.

That geographic divergence represents the stark separation in demographic responses to Trump's tumultuous presidency, with minorities, millennials and college-educated whites, especially women, recoiling from him in large numbers and blue-collar, older and evangelical whites providing him robust, even record, levels of support.

Not a wave, a realignment

In an NPR/Marist Poll released late last week, for instance, Democrats led on the "generic" ballot for Congress by 76 percentage points among African-Americans, 42 among Latinos, 35 among 18- to 29-year-olds and 9 among whites holding at least four-year college degrees. Republicans led by 56 percentage points among white evangelical Christians, 28 among rural residents and 21 among whites with less than four-year college degrees. College-educated white women preferred Democrats by 18 percentage points; non-college white men backed Republicans by 33 points.

"It's part of the sorting of the parties more by demographic characteristics, education being a very important one," says Gary C. Jacobson, a professor of political science emeritus at the University of California at San Diego who specializes in Congress. "The whole story of this election is Trump and how he affects voters. He clearly has driven away educated voters, especially educated women, with his style. Rural people, blue-collar people, don't mind it so much. They cut him some slack because they think he is on their side."

Similarly, longtime Democratic pollster Stanley B. Greenberg argues that those debating whether this election will produce a Democratic "wave" are analyzing it through the wrong lens. It's more likely, he says, that this election, punctuated by unusually high turnout, produces a realignment in which the groups alienated from Trump -- led by college white women, minorities and millennials -- consolidate around Democrats just as the groups that favor him, such as blue-collar and evangelical whites, consolidated behind Trump in 2016.

"I think he got his realignment ... but now we are seeing the reaction to that," Greenberg said.

To analyze the House battlefield CNN senior political producer Aaron Kessler used data from the Census Bureau's American Community Survey five-year average to create a demographic profile of each House district. Each district was measured on whether it ranked above or below the national average across a series of key characteristics, including education and income levels, the share of the population that is nonwhite and the median age.

Then we compared those results with the latest CNN ranking of the 96 House districts considered most likely to switch parties (that includes 95 seats described as toss-up, lean or likely toward either party, and one Republican-held seat now considered "solid" for Democrats). That analysis produced several distinct patterns.

Democrats are best-positioned in wealthier, more educated districts that didn't like Trump in 2016

The most striking is the concentration of the Democrats' very best chances in relatively affluent and well-educated districts. CNN rates 14 seats now held by Republicans as solid, likely or leaning toward the Democrats next week. The median income exceeds the national average of $55,322 in all of those except the Tucson-area seat being vacated by Arizona Republican Martha McSally (who's running for the Senate), where former Democratic Rep. Ann Kirkpatrick is a strong favorite.

The share of adults with college degrees exceeds the national average of about 30.3% in all of them except the New Jersey seat left open by retiring Republican Rep. Frank LoBiondo (where Democrat Jeff Van Drew is highly favored because Republicans nominated a fringe candidate who has expressed racist sentiments) and the northeast Iowa seat where Democrat Abby Finkenauer is mounting a strong challenge to Rep. Rod Blum.

The other seats leaning toward Democrats that currently are held by Republicans are all in districts that are relatively better educated and affluent. They include Rep. Mike Coffman's suburban Denver seat, Barbara Comstock's seat in Northern Virginia, the suburban Minneapolis districts of Reps. Erik Paulsen and Jason Lewis and other open seats in New Jersey, Pennsylvania, Michigan and Orange County, California.

Of these 14 GOP districts that CNN classifies as most likely to flip, eight of them voted for Hillary Clinton over Trump in 2016. Trump won each of the others by 5 points or fewer.

Beyond these 14 Republican-held seats that CNN rates as leaning to the Democrats, control of the House will likely be decided in 29 more GOP-held seats CNN classifies as toss-ups.

These also lean toward better-educated and more affluent districts, though not as lopsidedly as the Democrats' best chances. Of the 29 Republican-held districts considered toss-ups, 18 exceed the average median income and 17 exceed the average education level. In 2016, Clinton carried 13 of these 29 Republican toss-up districts.

In all, that means of the 43 Republican-held seats that CNN considers at greatest risk of falling to the Democrats, 31 (or nearly three-fourths) exceed the median income, while 29 (or just over two-thirds) exceed the average education level.

The Republican-held toss-up seats divide into two sharply delineated categories. The largest group generally resembles the affluent, well-educated suburban GOP seats where the Democrats' best chances are concentrated. These include open seats outside Seattle; Charlotte, North Carolina; and in Orange County, California; and challenges to Republican incumbents Mimi Walters and Dana Rohrabacher also in Orange County; Dave Brat in Richmond, Virginia; Peter Roskam outside Chicago; Mike Bishop near Detroit; Leonard Lance in New Jersey; Brian Fitzpatrick near Philadelphia; John Culberson near Houston; David Young near Des Moines, Iowa; Kevin Yoder outside Kansas City, Missouri; and a rematch between Republican Rep. Troy Balderson and Democrat Danny O'Connor after Balderson narrowly won last summer's closely watched special election for a district around Columbus, Ohio.

Many of the remaining GOP-held seats CNN rates as toss-ups will test the Democrats' ability to make gains on Trump's strongest ground: mostly white, heavily blue-collar areas. These include challenges to Republican incumbents Bruce Poliquin in Maine, John Faso and Claudia Tenney in New York, Mike Bost in Illinois, Scott Taylor in Virginia and an open seat in Kansas.

The toss-up seats also mark the first battlegrounds where the Democratic advantages among minority voters may meaningfully affect the battle for House control.

The most competitive districts are more white than the national average

One of the most striking aspects of the 2018 landscape is how few of the highly contested seats contain large numbers of nonwhite voters. Of the 14 Republican-held seats that CNN rates as tilting toward Democrats, the minority share of the population exceeds the national average of about 38% in just the Southern California open seat being vacated by retiring Rep. Darrell Issa.

In the 29 Republican-held toss-up seats, minorities exceed their national share of the population in just 11. Those seats are entirely situated across the Sun Belt: challenges to Republican incumbents Walters, Jeff Denham, Steve Knight and Rohrabacher in California; Carlos Curbelo in Miami; Culberson in Houston; and open seats in California, Florida, New Mexico and North Carolina.

The relative shortage of competitive seats with substantial minority populations underscores the structural hurdle Democrats face in the House from the concentration of nonwhite voters inside large metropolitan areas where the party is already strong. Already, about two-thirds of the seats Democrats currently hold have more minority voters than the national average, while fully four-fifths of the GOP-held seats are more white than the national average.

Compounding the Democrats' challenge this year, Hispanics represent the largest minority population in almost all the competitive Republican-held districts (North Carolina excepted) and Democrats remain concerned that turnout among them won't be nearly as high as they have hoped after all the provocations from Trump.

The battlegrounds are also older than the rest of the country

The nature of the House battlefield also dilutes another Democratic advantage: the party's edge among younger voters, particularly in the Trump era.

The NPR/Marist Poll released late last week, for instance, found that exactly two-thirds of those ages 18 to 29 disapproved of Trump's performance, and nearly as many indicated they intend to vote for Democrats. But the CNN demographic analysis found that the median age exceeds the national level (almost 38 years of age) in 30 of the 43 Republican-held districts rated as toss-ups or better for the Democrats.

That's a stark contrast with the House districts Democrats now hold, almost three-fifths of which are younger than the national average, largely because of the party's reliance on minority-centered districts with many children and young adults. (Three-fifths of current Republican seats are older than the national average.)

"Democrats have a serious structural problem in where these people are located," notes Jacobson. "Gerrymandering is a small part of the story; it is much more where people choose to live. If you look at Democratic demographics, African-Americans, Hispanics, young people, gay people, they all hang out in cities. They are attracted to big metropolitan areas."

The remaining two tiers of Republican-held seats at the periphery of the Democrats' target list would require the party to push even further into blue-collar and Trump-friendly territory.

CNN rates 20 Republican-held seats as leaning toward the GOP, which means they are competitive but Republicans are favored to hold them. Of these, 16 have fewer minorities than average, 13 are older than average, 10 are below the average education level and nine lag the median income. In 2016, Trump carried all but two of these districts (the seats near San Antonio and Dallas held by Reps. Will Hurd and Pete Sessions, respectively).

CNN classifies another 25 Republican-held seats as likely Republican, meaning that they are still potentially competitive but are considered long-shot chances for Democrats. Of these, 20 have fewer minorities than average, 18 are older than the national average, 15 lag the average education level and 14 trail the national median income. Trump won all of them except the two held by Republicans David Valadao in California's Central Valley and John Katko in upstate New York.

Still, for all these obstacles, the history of big midterm elections -- like 1994, 2006 and 2010 -- is that the party out of the White House usually wins some seats that had largely been considered safe. That's often because the incumbent takes the contest much less seriously than their counterparts in the toss-up districts. Republican Rep. Rob Woodall's seat in Georgia's Atlanta suburbs, now classified as likely Republican but drawing late interest from both sides, is one that could fit that description this year. Late Republican ad buys in unexpected seats, such as a district in South Carolina, raise that possibility too.

The only Democratic-held seats that CNN rates as toss-up or better for the GOP are two open seats in Minnesota: Both districts are older, more white and less educated than the national average. Six other Democratic-held seats are considered competitive but leaning or likely for the party to hold: Of these, four are above the national average in diversity, and four are below the average median income. Four are better-educated than the average.

All these patterns point toward the same conclusion: Democrats can likely win back the House, albeit with little margin for error, primarily by exploiting white-collar unease with Trump. But to generate big gains, they will also need to overcome the formidable defenses that Trump and the GOP have built beyond the nation's largest metropolitan areas.