By Ronald Brownstein, CNN

(CNN) -- For all of the attention to this year's House of Representatives special elections, November's gubernatorial races in New Jersey and especially Virginia will likely offer a more revealing measure of how Donald Trump's presidency is reconfiguring the political landscape.

Local issues and dynamics will strongly influence the results, but both contests are also illuminating the challenges that Trump's redefinition of the party carries for Republicans in affluent, diverse, heavily white-collar states, particularly those along the coasts.

With his bristling nationalist message and repeated gestures toward white racial resentments, Trump is generally strengthening the GOP's hand among culturally conservative working-class, evangelical and rural whites. But he's made those gains at the price of alienating more socially moderate suburban white-collar whites and deepening the resistance Republicans face among minority voters and Millennials.

The four 2017 special elections for open Republican House seats muffled the full implications of that trade-off because they all occurred on Republican-leaning terrain. Only the party's close call last June in the suburban Georgia seat vacated by (now former) Health and Human Services Secretary Tom Price hinted at the difficulties Trump's positioning could cause with upscale white voters who ordinarily lean right on economics but tilt toward the center-left on cultural questions and racially-tinged issues like immigration.

The governor race in New Jersey between Republican Kim Guadagno and Democrat Phil Murphy and especially the Virginia contest between Democrat Ralph Northam and Republican Ed Gillespie will offer a more complete balance sheet on Trump's impact. Both states' GOP gubernatorial nominees -- Guadagno, now the lieutenant governor and Gillespie, the former Republican National Committee chair -- have worked diligently to maintain distance from Trump. And yet polls show that voters' preferences in the governors' races are strongly tracking with their attitudes toward Trump. In particular, the antipathy toward Trump among college-educated white voters in both states has created a significant headwind for each Republican.

The Democratic contenders are also benefiting from the relative popularity of outgoing Democratic Gov. Terry McAuliffe in Virginia and the record unpopularity of outgoing Republican Gov. Chris Christie in New Jersey. One other factor boosts the Democrats too: except for McAuliffe's Virginia victory in 2013, in both states, the party that lost the White House the previous year has captured the governorship in each election since 1989.

Trump's impact is almost certainly less significant in New Jersey, though only because Christie's troubles have prevented Guadagno, a moderate, from ever becoming competitive. With Christie sinking to record-low approval ratings, polls have consistently shown Murphy, a wealthy former investment banker and ambassador to Germany, routing Guadagno in a race that has stirred strikingly little local interest.

"I think it's going to be fairly difficult ... to make a big judgment about Trump based on the results of the New Jersey gubernatorial election in part because it's going to be difficult to separate Trump's unpopularity vs. Christie's unpopularity, which has had a bigger impact on voters," says Kris Shields, an assistant research professor at Rutgers University.

Even with the Christie effect, though, recent polling has shown a close correlation between attitudes toward Trump and preferences in the gubernatorial race. In Quinnipiac University's most recent New Jersey poll, support for Guadagno closely tracked Trump approval among non-white voters (17% support for her, 16% Trump approval); independents (36% for her, 34% for him) and college-educated whites (36% for her, 34% for him.) While Christie comfortably carried college-educated whites in each of his two victories, the survey showed the Democrat Murphy now holding a wide lead among them.

In all, Quinnipiac found, three-fifths of New Jersey voters disapproved of Trump and 81% of them were supporting Murphy.

The Trump stamp on the GOP presents almost no upside for Republicans in New Jersey, a state with virtually no rural population, few evangelicals, and large concentrations of minority and white-collar white voters.

The equation is more complicated in Virginia. That's a state where large minority and white-collar populations, largely in the state's booming Northern Virginia suburbs outside Washington DC, are balanced (though not equaled) by substantial concentrations of rural and evangelical whites, mostly in the state's southern and western counties.

Though Democrats have taken the state in three consecutive presidential elections and hold both US Senate seats, Republicans still control the state legislature and held the governorship from 2009 through McAuliffe's win in 2013. (The Republican governor that served that term, Bob McDonnell, was the subject of a high-profile federal corruption trial after he left office.)

Gillespie's unusually complex campaign gyrations capture the challenge Republicans now face navigating their way through this landscape. On the one hand, Gillespie has doggedly refused to directly criticize Trump or directly disavow almost any of his policy initiatives. Gillespie didn't denounce Trump's widely denounced response to the Charlottesville violence and he has steadfastly refused to take a position on any of the serial Republican efforts to repeal the Affordable Care Act.

On the other, though Gillespie was previously considered a voice for racial inclusion in the GOP, he has placed a surprising emphasis on some Trump-like themes. Gillespie has opposed the removal of confederate monuments in the state (though he's said they should be balanced with more "historical context") and his television advertising has stressed his opposition to "sanctuary cities" that don't fully cooperate with federal immigration enforcement, and tried to link Northam to the violent Central American gang MS-13.

Operatives in both parties see these contrary, if not mixed, messages as a measure of how complicated it has become for Virginia Republicans to tightrope between a party base increasingly responsive to Trump-like racially-tinged populism and the rapid growth of diverse, socially moderate suburbs not only outside Washington DC but also Richmond. Trying to satisfy each, Gillespie risks disappointing both.

"A lot of the Trump people are saying he [Gillespie] is not really with us," says Larry Sabato, director of the Center for Politics at the University of Virginia. "And I don't know any Democrat who would vote for Gillespie simply because he's putting a little distance between himself and trump. I don't' think we have ever been this polarized in this lifetime."

One GOP strategist familiar with the Gillespie camp's thinking says one reason he has offered such complex messaging is that attitudes toward Trump have remarkably diverged across the state. "There is a boatload of people who think he's god-like, and then there are people who are very mediocre toward him -- in Richmond and the [southeastern] Tidewater region -- and then there's the greater Washington DC area, where he's nothing short of fingernails on the chalkboard," said one GOP strategist familiar with the Gillespie camp's thinking.

Northam, a pediatrician and former State Senator, is emphatically low-key as a candidate. But he has unequivocally denounced almost all of Trump's major initiatives and, beginning in the Democratic primary, he labeled the president a "narcissistic maniac."

Almost all public polls show Northam holding a small lead of around four or five points, and the race remains within reach for either side. But Gillespie faces two big hurdles, both tied to Trump. Historically, Republicans have benefited in these off-year Virginia gubernatorial elections from a significant decline in turnout among strongly Democratic African-American and younger voters, producing an older, whiter and more conservative electorate; one reason McAuliffe won is he minimized that fall-off.

Stephen Farnsworth, a political scientist at the University of Mary Washington, says the off-year tilt toward Republicans likely won't be as severe as usual again this year for three reasons: Democratic hostility to Trump; an increased number of competitive Democratic State House candidates inspired largely by that hostility who will work to turnout their party's vote; and the presence on the ticket of an African-American Lt. Governor nominee, Justin Fairfax.

As in New Jersey, Gillespie's other problem is unusual weakness among college-educated white voters. McDonnell won them comfortably in the 2009 Virginia gubernatorial election and in 2013 even staunch conservative Ken Cuccinelli narrowly carried them over McAuliffe (probably partly because of a backlash against the botched launch of the Obamacare website that fall, as Sabato notes.) Gillespie also won a 53% majority of college whites in his narrow 2014 Senate race loss to Mark Warner.

Several recent Virginia polls, though, show Trump sagging with those voters and dragging Gillespie down with him. The latest Virginia polls from Quinnipiac, the University of Mary Washington and Monmouth University all show Trump's approval rating among college-educated whites below 40 percent and Gillespie drawing only about 40% or slightly more of their votes. In each case, Northam leads with them. Geoff Garin, Northam's pollster, says that for both Trump and Gillespie the weakness is "especially dramatic among college educated white women in a way that could exceed [the Democratic margins] that happened with Hillary Clinton."

In all three surveys, Gillespie holds a commanding lead with the blue-collar whites that also boosted Trump. But given the Republican weakness with minority voters, that working-class white advantage isn't enough to win in Virginia if Democrats break even or better with white-collar whites, strategists in both parties agree.

The GOP strategist says that to win Gillespie must come close to matching his 45% showing from 2014 in the well-educated DC suburbs-a daunting challenge given that Trump's approval in the same area now stands at only about one-third. Statewide, in the Quinnipiac, University of Mary Washington and Monmouth surveys, Trump's disapproval rating stood at 55% or more-and Gillespie was winning fewer than one in 10 of the voters who disapproved of Trump in each poll. That's a deep hole to overcome.

Given Virginia's underlying trends, a Gillespie defeat wouldn't be shocking -- and a Guadagno defeat would be even less surprising amid the wreckage Christie has left in New Jersey. But if minorities and perhaps Millennials exceed their usual off-year turnout in the two states and college-educated whites migrate further away from the GOP, Republicans will have even more reason for biting their nails about Trump's impact on 2018.