By Ashley Killough

CNN

WASHINGTON (CNN) -- A total of 10 Senate Democrats are up for re-election next year in states won by President Donald Trump.

Half of those senators are considered especially vulnerable because they represent states viewed as reliably red or states that have grown more Republican in recent years.

Those include Sens. Joe Manchin of West Virginia, Heidi Heitkamp of North Dakota, Joe Donnelly of Indiana, Claire McCaskill of Missouri and Jon Tester of Montana.

The other half hail from swing states that Trump won by a smaller margin and are less predictable.

Political observers will keep a close eye on this first group of senators and their actions on Capitol Hill. Will they support the President's agenda? Or will they side with Democrats in big votes? Will they act as deal-makers between the two parties?

In the first 100 days of the Trump administration, several have made White House visits to sit down in a group setting with the President or attend bill signings.

Three of those Democrats --- Manchin, Heitkamp and Donnelly -- went against their own party's filibuster of Supreme Court nominee Neil Gorsuch and ultimately joined Republicans in the final confirmation vote.

This group of Democrats also had the fewest "no" votes on other major Trump nominees.

Meanwhile others, like McCaskill and Tester, refused to advance Gorsuch's nomination and voted against his final confirmation.

As for legislation, a few of the senators have crossed party lines to vote with Republicans this year, including on a bill that would roll back Obama-era regulations to prevent coal mine waste from entering waterways to keep guns out of the hands of certain people with mental health issues.

One thing they all share is ramped-up pressure from outside groups on the right and the left.

Republican groups targeted them in the Gorsuch fight, releasing ads in their Trump-won states urging them to advance Gorsuch's nomination.

And while many of them have tried to position themselves as centrist or moderate Democrats, Republicans have begun working to link them to liberal Democratic Sen. Elizabeth Warren.

They've also faced opposition from activists on the far left who have threatened primary challenges, though no serious challengers have stepped forward yet.

Here's a look at what the five senators have done so far this year:

Joe Manchin, D-West Virginia

Manchin has long been pushing his brand as a centrist Democrat willing to work across the aisle. That came into full focus this year after Trump won West Virginia overwhelmingly (by 42 points) in the November election.

The senator met with Trump in December to discuss a potential spot as energy secretary, but soon after the meeting Manchin said he planned to stay in the Senate.

Since then, Manchin has visited the White House multiple times and told CNN last week he has a "good relationship" with Trump.

"I have access, I can speak to him," he said. "I don't think he looks at me from the standpoint, 'Well, you're the opposition party and you're the opposition.'"

Manchin has voted against four of Trump's Cabinet or major administration nominees -- casting the fewest "no" votes among Democrats -- and was the only one in his party to vote in favor of confirming Jeff Sessions for attorney general.

Manchin also worked behind the scenes with a bipartisan group of senators to urge both parties to find a deal to avert the "nuclear option" in the battle over Gorsuch.

The talks were unsuccessful, but Manchin was vocal in his disappointment with both sides. He was one of three Democrats who ultimately voted to confirm Gorsuch.

In an unusual move, the senator has also been getting to know conservative media outlets, including Breitbart.

Manchin, who's in his first full term as a senator, faces a tough re-election battle in West Virginia next year, where Republicans feel confident they can pick up a Senate seat, given the state's support for Trump last year. Unlike other Democrats, who've benefited from grassroots enthusiasm, Manchin only raised about $550,000 in the first quarter.

Still, a Democrat, Jim Justice, was elected governor of West Virginia on the same ballot as Trump, showing the state's not-so-predictable voting behavior.

Rep. Evan Jenkins is considering a run for the Republican nomination to challenge Manchin, as is state Attorney General Patrick Morrisey.

But Manchin, who's also served as governor of West Virginia, is a well-known figure in the state, and he's relying on his brand as a bipartisan broker to carry him to a second term.

Heidi Heitkamp, D -- North Dakota

Heitkamp was first elected in 2012 and raked in $1.6 million this quarter, finishing out the first three months of the year with $2 million in cash on hand.

Like Manchin, she also comes from a state where Trump crushed Hillary Clinton last year, by 36 points.

Heitkamp was another Democrat who voted to confirm Gorsuch to the Supreme Court, and she voted against five of Trump's nominees.

She also met with Trump during the transition about a potential Cabinet post and went to the White House with other senators for a lunch in February and separately for two bill signings.

Rep. Kevin Cramer, North Dakota's at-large representative, is thinking about challenging Heitkamp next year. But CNN's Manu Raju reported earlier this month that Senate GOP officials have grown concerned that Cramer's penchant for controversial remarks could damage the party's chances.

Heitkamp is heavily involved in tribal issues and introduced a bill this year to help Native American children who have experienced trauma. She's also paired up with Republican senators like Dean Heller of Nevada, Roger Wicker of Mississippi and John Boozman of Arkansas on other bills.

Joe Donnelly, D-Indiana

Donnelly also was first elected to the Senate in 2012, following three terms as a congressman. Trump won the state of Indiana, where Vice President Mike Pence was recently the state's governor, by 19 points.

Donnelly was the third Democrat to back Gorsuch, joining Manchin and Heitkamp, and he voted against six of Trump's major nominees. (He did not cast a vote on the nomination of Scott Pruitt for EPA administrator.)

He was also part of the bipartisan group of senators that met with Trump at the White House in February about Gorsuch, along with Manchin, Heitkamp and Tester.

Donnelly raised $1.3 million in the first quarter for 2018 and has $2.5 million cash on hand.

Indiana Reps. Luke Messer and Todd Rokita are among the Republicans who could challenge Donnelly.

He paired up with the junior senator from Indiana, Republican Todd Young, this year to introduce legislation supporting mental health services for police officers. He also helped introduce a bill with other Democrats to end outsourcing.

Claire McCaskill, D-Missouri

McCaskill, facing pressure from both sides of the aisle, has also been trying to paint herself as a moderate. She's up for a third term next year.

Republicans consider it a potentially strong pick-up seat, but there are some signs of advantages for McCaskill. While Trump won the state of Missouri by 19 points, the incumbent Republican senator, Roy Blunt, won by a far smaller margin of only three points on the same ballot.

And McCaskill raised a whopping $2.8 million in the first quarter and has $3 million in the bank.

At a town-hall style meeting this month in Missouri, McCaskill was asked if she thinks the state's large number of Trump voters will hurt her chances at re-election, according to the New York Times.

"I'll tell you one thing that drives me crazy: my colleagues in Washington that are from places like California and New York who wanted to look down their nose at Trump voters," McCaskill said. "I think that's a huge mistake."

At the same time, McCaskill, who has publicly acknowledged that she could face a primary challenger, has been critical of several of Trump's policies.

While McCaskill expressed concern about the use of the nuclear option to confirm Gorsuch, she ultimately joined her party in the filibuster and opposed his confirmation.

She also voted against seven of Trump's nominees and did not vote on Tom Price's nomination for Health and Human Services secretary.

McCaskill has helped introduce multiple bills this session aimed at eliminating waste in government. She's the top Democrat on the Homeland Security and Governmental Affairs Committee.

Jon Tester, D-Montana

Tester, who was first elected to the Senate in 2006, is up for a third term next year in Montana, which Trump won by 21 points. Still, in a sign of hope for Tester, the state's Democratic governor, Steve Bullock, also won statewide election on the same ballot last November.

Like McCaskill, Tester opposed confirmation of Gorsuch, saying he didn't feel like Gorsuch was strong enough on campaign finance issues. He also voted against eight of Trump's other major nominees.

Tester showed a strong first quarter, bringing in $2 million and ending with $3.16 million cash on hand.

Trump signed a bill into law that Tester, the ranking member on the Veterans Affairs Committee, introduced earlier this year to expand and reform the Veterans Choice Program, which allows some veterans to use provide health care providers located near their homes. The bill was cosponsored by three Republican senators: John McCain, Johnny Isakson and Jerry Moran.

Montana state Republican Sen. Albert Olszewski announced last week that he plans to challenge Tester, according to local reports.

Previously the state's at large congressman, Ryan Zinke, was considered the most likely Republican to challenge Tester, but he was tapped by Trump for secretary of the Interior Department.