Sen. Robert P. Casey Jr. (D-Pa.) talks to a union member after a meeting with Sheet Metal Workers Local 44 in Wilkes-Barre, Pa., on Aug. 14. (Melina Mara/The Washington Post)

Democrats facing reelection next year in states President Trump won are seizing on trade at this early stage as a crucial issue and a Republican vulnerability.

But rather than jeer Trump’s protectionist positions, Democrats are echoing them and amplifying them, arguing that Trump has failed to fulfill his dramatic campaign promise to rip apart trade deals.

“When we say renegotiating NAFTA, we mean a transformation, something substantial, not just going through the motions,” Sen. Robert P. Casey Jr. (D-Pa.) told union leaders recently, referring to the administration’s talks over the North American Free Trade Agreement.

For Democrats, Casey’s pitch signals a wholehearted revival of their labor roots and a sharp departure from the free-trade tilt of the past two Democratic presidents, Barack Obama and Bill Clinton.

The change may also be necessary. While progressive activists are highly motivated against Trump and some suburban voters are increasingly uneasy about Trump’s disposition, many working-class voters remain resentful of globalization and have lingering appreciation for Trump’s populist appeals.

Sen. Robert P. Casey Jr. D-Pa.), center, meets with Sheet Metal Workers Local 44 during a trade forum in Wilkes-Barre, Pa., on Aug. 14. (Melina Mara/The Washington Post)

To win them back, Casey is making regular visits to Rust Belt cities, including Wilkes-Barre, which sits in Luzerne County in northeastern Pennsylvania. Both Luzerne and Pennsylvania went for Trump in the 2016 presidential election after more than two decades as Democratic strongholds.

“When China cheats, Pennsylvania loses jobs,” Casey told the group — all white men — at Local 44, a sheet-metal workers’ outpost. “We have to be much tougher in going after the cheaters of the world.”

Casey is following a playbook laid out by Senate Minority Leader Charles E. Schumer (D-N.Y.). As Schumer stares down a difficult 2018 election map for Democrats and tries to upend Republicans’ 52-seat control of the Senate, he has urged his colleagues to bring trade to the fore.

“Nothing, nothing is more central,” Schumer said last month at a news conference announcing proposals that included creating a “trade prosecutor” to supplement the work of the U.S. trade representative and a bevy of measures to stifle foreign competition and block overseas investments.

Schumer added, “The problem is President Trump has talked a good game and done virtually nothing on trade but study it. . . . We need action.”

As Schumer called China a “rapacious” trade partner, standing alongside the New Yorker were several Democratic senators running next year in states Trump carried: Casey, Joe Manchin III (W.Va.), Joe Donnelly (Ind.), Tammy Baldwin (Wis.), and Debbie Stabenow (Mich.).

House Democrats, who see a chance to take back control of their chamber if Trump’s approval rating sinks, are also on board. Last month their leadership and Schumer jointly unveiled a “Better Deal” agenda that pledged to “crack down” on trade practices.

The effort has been countered by Republicans. Donnelly, in particular, has been targeted by the National Republican Senatorial Committee as “Mexico Joe” over his past work for a family-run business that manufactured products in that country. (Donnelly has since sold his stock in the company.)

Donnelly met late last month with steelworkers in Indiana, a state Trump won by 19 percentage points, to assure them he was on their side. “I voted against every bad trade deal that’s come along,” he said.

But the GOP response beyond opposition research and allegations of hypocrisy has been scattered. Republican congressional leaders remain leery of Trump’s eagerness to impose tariffs and continue to embrace free trade.

“I’m a little concerned about some of the trade rhetoric, not only by the president, who succeeded, but by the people who were running against him,” Senate Majority Leader Mitch McConnell (R-Ky.) said last month at the Kentucky State Fair. “We still have a selling job to most Americans that trade is a winner for America. We’ve been a great trading nation going back to the founding of the country.”

Democrats’ push comes as Trump struggles to follow through on his ambitions to overhaul trade policy.

Early moves by the administration — withdrawing the United States from the Trans-Pacific Partnership in January and appointing Robert E. Lighthizer, who has a protectionist bent, as U.S. trade representative — have been followed by a string of fits and starts.

Trump authorized Lighthizer last month to consider investigating China’s trade policies on intellectual property, but it could take a year for a decision on a formal probe to be made. Proposed steel tariffs have stalled amid squabbles, and Trump’s manufacturing council was disbanded in the wake of an uproar over the president’s equivocal response to violence during rallies by white supremacists in Charlottesville.

Inside the White House, there is tumult over the direction of policy.

Stephen K. Bannon, who was the administration’s ardent advocate for nationalist trade policies, has left his position as Trump’s chief strategist. National Economic Council Director Gary Cohn, a former president of Goldman Sachs, has become a powerful force in the West Wing but has expressed disapproval of the president’s hard-line instincts on trade, according to administration officials who spoke on the condition of anonymity to describe private conversations.

Trump has also fumed about the slow pace on trade and complained to aides that U.S. trade rivals have taken notice of how one of his signature issues has become mired by deliberation, the officials said.

Meanwhile, the outcome of the ongoing NAFTA negotiations with Mexico and Canada, which began their second round Friday after months of turbulence, is unclear. Trump tweeted last week: “both being very difficult, may have to terminate?”

Democrats see the murky future of Trump’s trade planks as an opportunity to reclaim the issue as their own.

Casey, 57, who has a gentle cadence and has long kept a low profile in the Senate, may be showing the way.

As Casey prepares to run for a third term, he has stepped out on Twitter with scathing messages about Trump that have given him fame of sorts with younger voters and with the Democratic base. His town halls have drawn big crowds of college students, and well-educated Trump critics like him.

Casey, however, is not counting on riding the Trump resistance alone to reelection. Rep. Lou Barletta (R-Pa.), a Trump supporter who has built a national following because of his deeply conservative views on immigration, announced a Senate run late last month, probably counting on Trump’s enthusiastic support should he win the Republican nomination.

So when he is not in Washington, Casey and his staffers can be found riding around Pennsylvania to huddle with union leaders and meet with elderly voters — many of them retired union members. On one recent weekday, he hustled from a packed forum on aging to the Wilkes-Barre union hall and then to a tour of infrastructure projects at Scranton’s airport, followed by handshakes at a happy hour.

“A lot of the votes that Trump got were from Democratic voters,” Casey said in an interview. “You had to ask yourself, ‘Why is that? Why did that happen?’ You just get ready to get to places where you may not be on a regular basis. We’ve been going to a lot of small-town and rural areas over the last seven, eight months.”

It is not a campaign yet, but it has the feel of a summer test run.

And after years of delicately commenting on trade during the Obama presidency as the president pursued trade deals — he was a reliable Obama ally — Casey is back to being what he was at the start of his career and what his late father, the former governor Robert P. Casey Sr., was before him: an unabashed champion of labor.

“I know it’s been awfully difficult. If there is one group of Americans the right has targeted for years, it’s the men and women of organized labor,” Casey told the men at Local 44. “We’ve got a long way to go fighting back against those forces and at the same time raising wages, and all of that is undermined by bad trade deals.”

Left mostly unmentioned during Casey’s stops throughout the day and in his small talks with voters were the federal investigations into Russian interference in last year’s election. The culture wars were dormant. Casey is a Roman Catholic who opposes abortion, and that issue has roiled his past races, but no one brought it up.

The reception at the union gathering for Casey, lanky and clad in a blazer and loafers among burly men in T-shirts and jeans, spanned from polite to warm. He listened intently, never raised his voice and said what they wanted to hear.

“I’m getting tired of these promises from Trump,” Joe Padavan of the United Steelworkers told a nodding Casey. “We’re losing health care, we’re losing pensions, and all products should be made in America.”

There was a flash of awkwardness. When Warren Faust, who works in construction, mentioned an ethnic slur that some workers use to describe metal made in China, Casey did not correct him.

Later, at lunch nearby, Casey said he wished he had spoken up.

“It’s difficult,” he said. “We all have to be vigilant about that. Maybe I should have said something to him.”

The moment was nonetheless telling. As Democrats go all in on trade, they also are navigating minefields of grievance that have only expanded since Trump took office, and grievance can be ugly.

But mine they must.