Sen. Joe Donnelly (D-Ind.) takes questions in Anderson, Ind., as part of his “Hoosier Highways” RV tour. (Don Knight/Herald-Bulletin/Associated Press)

The worker’s message for Sen. Joe Donnelly was blunt: “President Trump, I think, has got a long road ahead of him,” he said, “and I hope you can back him on some things, because I think he has the country’s best interest at heart.”

Donnelly assured the man that he is working with Trump on creating jobs and combating ­opioid abuse. Looking out on the construction of a railroad underpass in this rural town, he said, “Anywhere we’ve got some ­common-sense stuff, count me in.”

“I hope so,” said the worker, who declined to give his name to a reporter. “Because if not, then I’m going to be voting for someone else.”

It’s a message Donnelly, a burly Indiana Democrat, is hearing a lot. It’s one he and other Democrats seeking reelection next year in states Trump won are responding to in a way that puts them at odds with the leadership and base of their own party: by promising to work with this president.

Donnelly is one of 10 such Democratic senators — and one of five in states where Trump’s margin entered the double digits. (In Indiana, Trump won by 19 points.) They face a seemingly impossible challenge: to appeal to Trump voters while retaining the support of anti-Trump Democrats. They must do so not only for their own political futures but for their party to win back control of the Senate and compete in states rich with the white ­working-class voters who drifted to Trump in 2016.

Donnelly, second from left, poses for a photograph with a supporter at Savage's Ale House in Muncie, Ind. More than a year before the vote, his reelection bid is rated a toss-up. (Mike DeBonis/The Washington Post)

Key congressional forecasters consider Donnelly’s race a toss-up, even without a clear Republican opponent. The same goes for the races of Democratic senators in Missouri, Montana, North Dakota, West Virginia and other states — all caught between their constituents’ support for Trump and a national Democratic brand increasingly centered on resistance to the president.

It’s a delicate dance for many of them. Sen. Claire McCaskill (Mo.) described her approach at a recent town hall: “My job isn’t to fight the president. My job is to fight for you.” It was on full display last week as Donnelly kicked off his reelection campaign, touring the state in an Indiana-built RV.

At the highway project, Donnelly basked in the praise of the local mayor, who said that if the senator hadn’t made a phone call to the railroad, the project might still be entangled in red tape.

That’s the kind of prudent, results-oriented governing Donnelly ran on when he pulled off his unlikely 2012 victory, and it’s the image he hopes voters will have in mind when they cast their ballots next year. But he is frequently reminded that his reelection isn’t going to be about just roads and bridges.

A few hours before inspecting the underpass, Donnelly rose in a union hall in the factory suburb of Anderson and denounced the violent white-nationalist rally in Charlottesville — and, by implication, the president’s tepid response to it.

“There is no place for white supremacy, for neo-Nazis, for KKK in the United States of America,” Donnelly declared to a standing ovation. He later told reporters that Trump’s remarks were “way off the mark” and that he hoped the president “might choose his words more wisely on those types of issues.”

Donnelly speaks at the UAW in Anderson, Ind., as he starts his reelection campaign. (Don Knight/Associated Press)

That’s as far as Donnelly went — and he otherwise steered clear of open conflict with Trump, touting instead his own bipartisan initiatives to crack down on corporate outsourcing, address the opioid epidemic and beef up military readiness and services for veterans.

In an interview, Donnelly relayed what he’s heard from Indiana voters over the past eight months: “ ‘Be with him, and don’t try to give him a hard time just because he’s the president.’ That’s pretty universal.”

As much as any congressional Democrat, Donnelly has complied with those wishes. He has voted for more Trump Cabinet nominees than all but four other Democratic senators and was one of three Democrats to vote to confirm Neil M. Gorsuch as a Supreme Court justice. He also has sided with Republicans on some efforts to overturn environmental regulations.

Donnelly has kept his Washington profile low, ignoring reporters in the Capitol hallways and eschewing the cable news circuit, preferring instead a robust schedule of local TV and radio appearances back in Indiana.

But he also has been a team player for Democrats on most key issues — including preserving the Affordable Care Act. That gives GOP leaders in both Washington and Indiana what they say is a clear path to reclaiming his seat in 2018 by tying him to the national party.

“We know the Trump voters, and they are not Donnelly voters,” said Kyle Hupfer, chairman of the Indiana Republican Party. “The reality is, he’s not supportive of the president, and voters in Indiana are going to know that.”

Reps. Luke Messer and Todd Rokita are giving up their safe House seats to take on Donnelly, and five other Republicans have also declared their candidacies.

Donnelly’s air of vulnerability is partly rooted in the circumstances of his 2012 victory.

He won comfortably against Republican Richard Mourdock, then the state treasurer, whose campaign imploded after he suggested in a debate that a pregnancy resulting from a rape was “something that God intended to happen.”

But political observers in Indiana say it would be a mistake to dismiss Donnelly’s victory. They credited a skillful campaign that positioned the Democrat as a bipartisan problem-solver — including TV spots where he literally stood in the middle of a road to play up his centrist outlook. Current public polling is scarce, but strategists cite past surveys and anecdotal evidence indicating that Donnelly has built a genuine bipartisan following.

“Donnelly won by six points because of the rape comment, but he didn’t win because of the rape comment,” said Christine Matthews, who polled the 2012 race and has worked for numerous Republican candidates in Indiana.

Donnelly’s message is well calibrated to a state that is 85 percent white and has seen a decline in low-skill manufacturing jobs, and where three-quarters of the population has no more than a high school degree. It involves a major focus on the effects of foreign trade, highlighting his work on the Armed Services and Agriculture committees, and a hefty dose of reverence for service members and veterans.

In Winchester, he visited a facility for homeless veterans, where he pressed administrators and local officials on how the federal government could do more to help them and quizzed the residents on the challenges they faced.

Greg Beumer, a Republican state lawmaker, attended the event and had little ill to say about Donnelly.

Many Republicans were “scratching their heads,” he said, at why Messer and Rokita would risk safe seats to take on Donnelly, “who by and large has done a very good job of being that moderate voice in the Senate.”

Beumer wouldn’t say whether he would vote for the Democrat, but he said moderate Republican voters will be “up for grabs” next year. “I would say if I were in his shoes, he’s done exactly what I would have done,” Beumer said.

Donnelly has been most outspoken on economic issues, introducing an End Outsourcing Act and waging war against Carrier Corp.’s decision to move hundreds of jobs from an Indianapolis furnace factory to Mexico — a crusade Trump later joined, culminating in a dramatic announcement in November that the then-president-elect had brokered a deal to keep most of the jobs intact.

Those bombastic promises did not completely pan out — Carrier began laying off more than 600 workers last month — but while other Democrats have lambasted Trump for a bait-and-switch, Donnelly doesn’t. “There’s more jobs that stayed after he got involved than there was before he got involved,” he said.

Donnelly has taken a recent bruising on the outsourcing issue, thanks to an Associated Press report in July that revealed that an arts-and-crafts company owned by the Donnelly family uses Mexican labor for some of its products. The senator owns a minority stake in the company.

The AP reported this week that Donnelly is finalizing the sale of his share of the company, Stewart Superior Corp., and donating the proceeds to charity, but the political toll has come swiftly: The National Republican Senatorial Committee has already started trying to brand Donnelly as “Mexico Joe.”

It hired a mariachi band to play outside his campaign kickoff in Anderson last week.

In an interview, Donnelly said he has had no active role in the company in two decades and he learned of the outsourcing only when he was contacted for the AP report. “The subject never came up” in conversations with family, he said, adding, “I was disappointed when I found out.”

Donnelly noted that a company facility in Indiana has added jobs in recent years and said his record on trade speaks for itself: “I’m more than happy to have this discussion any place, any time.”

For now, it is Trump — not outsourcing — that appears to weigh more heavily on many Hoosiers’ minds. Kaye Whitehead, a farmer and former Republican county party chairman, attended a Donnelly campaign event on a Hartford City farm to hear his thoughts on the upcoming farm bill — and on Trump.

“We need to support the president of the United States no matter who it is,” Whitehead said. “I really think he’s got some good ideas, but nobody gets on board.”

As the sun beat down on the cornfield outside, Donnelly stood inside a pole barn and rattled through his efforts to maintain crop insurance programs and his opposition to an Obama administration regulation that applied Clean Water Act protections to small streams.

Whitehead listened closely and said afterward that she was willing to give Donnelly a chance. But she said she would be watching closely, with a particular eye on health care and taxes.

“There’s a lot of issues on the table that the senator will have to vote for or against, and I’m interested in seeing how he proceeds.”