DETROIT — The Democratic presidential candidates have been chasing labor support all summer, appearing at small union halls and large conferences, and tweeting support for workers at companies like Amazon and Walmart. But now, as the United Automobile Workers, one of the nation’s largest unions, stages a strike that has even drawn words of support from President Trump, Democrats are seizing the moment to align themselves with workers in public and dramatic ways.
Senator Elizabeth Warren of Massachusetts walked the picket line Sunday alongside striking General Motors workers at an assembly plant in Detroit. Not to be outdone, former Vice President Joseph R. Biden Jr. appeared at another G.M. assembly plant in Kansas City, Kan.
The picket line visits of two of the leading candidates for the Democratic nomination — with the third, Senator Bernie Sanders of Vermont, planning to join striking workers in Detroit on Wednesday — illustrated the importance to Democrats of winning the support of rank-and-file union members, including those who voted for Mr. Trump in 2016.
Taking her message of fighting inequality to Michigan for the fourth time since June, Ms. Warren joined a scrum of striking autoworkers, carrying a blue and white “U.A.W. on Strike” sign. As they crisscrossed the entrance to the Detroit-Hamtramck Assembly Plant, where Cadillacs and Chevrolets are built, the workers chanted, “We are the union, the mighty, mighty union. U.A.W. stand strong.”
Ms. Warren criticized G.M. for closing plants while making billions of dollars in profits. “G.M. is demonstrating that it has no loyalty to the workers of America or the people of America,” she said. “Their only loyalty is to their own bottom line. And if they can save a nickel by moving a job to Mexico or to Asia or to anywhere else on this planet, they will do it.”
“Everybody deserves a living wage in this country,” she said. “Let’s be clear, unions built America’s middle class and unions will rebuild America’s middle class.”
U.A.W. leaders in Detroit voted unanimously a week ago to authorize the strike, the union’s first such walkout since 2007. The union is pushing G.M. to raise wages, reopen some plants and add jobs at others, and narrow the pay gap between new hires and veteran workers. Nearly 50,000 members have joined picket lines at factories across the South and the Midwest.
Every major Democratic presidential candidate has expressed support for the striking workers, and several have visited picket lines to appeal to union members who may have switched their support to Mr. Trump. Senator Amy Klobuchar of Minnesota joined the one in Detroit on Thursday, while Representative Tim Ryan of Ohio traveled to several in his home state and in Flint, Mich.
Addressing the striking U.A.W. workers in Kansas City on Sunday afternoon, Mr. Biden wore a red T-shirt in solidarity. He referenced the bailout of General Motors during the Obama administration, telling the crowd, “We didn’t bail out GM, U.A.W. bailed out GM!” He went on to lament the high pay of executives at the company and the lack of equitable benefits for workers, and he encouraged the striking workers, acknowledging, “you’re making a hell of a sacrifice.”
“There’s only one reason we have a middle class, and it’s spelled ‘U-N-I-O-N,’” he said. The crowd cheered.
Auto-worker support for the Democratic Party has eroded slightly in recent years, and about 30 percent of the union’s rank and file were estimated to have voted for Mr. Trump in 2016, slightly more than the share who voted for the two previous Republican presidential nominees.
The departure of those voters from the Democratic Party, even as the union had officially endorsed Hillary Clinton, was particularly critical to Mr. Trump in Michigan, where he won by the thinnest of margins — less than 11,000 votes — to capture the state’s 16 electoral votes.
Nearly three years later, it is unclear how many of those union workers continue to back the president. The economy has generally remained strong here, with unemployment near a 20-year low.
Representative Debbie Dingell, a Democrat who represents a district just outside of Detroit that is home to several auto manufacturing facilities, said she felt that Mr. Trump’s support among autoworkers had held roughly steady.
But she said she believed that a prolonged strike could move the president’s numbers among such workers in one direction or the other, depending on whether they viewed him as trying to help or hurt them.
“If he’s perceived as trying to keep the plants open, and it’s successful, it could have a positive impact,” Ms. Dingell said.
In 2016, Ms. Dingell was a lonely voice among Michigan Democrats as she raised concerns that Mr. Trump could carry the state, even going so far as to hold a meeting with Democratic legislators in her district several weeks before the election warning them not to underestimate his appeal.
Ms. Dingell said she had heard frequently from autoworkers at dozens of community meetings in 2016 that they were “with Donald” and that they believed he sincerely cared about trade.
Mr. Trump last week expressed hope that the strike would be a “quick one,” complaining about General Motors’ decision to close domestic plants and open factories offshore, and taking a swipe at the company’s leadership, which announced last November that the company would close four plants in the United States.
“I got tremendous numbers of votes from the autoworkers,” Mr. Trump told reporters at the White House. “I don’t want General Motors to be building plants outside of this country. You know they built many plants in China and Mexico and I don’t like that at all.”
While Mr. Trump ran as a friend of workers, some labor leaders have said that was largely empty rhetoric. In a recent interview on Fox News, the president of the A.F.L.-C.I.O., Richard Trumka, said workers were suffering, and criticized the White House’s inaction on the minimum wage. During the 2016 campaign, Mr. Trump had said he supported hiking the federal minimum wage, currently at $7.25 an hour, to $10.
“He’s opposed every increase in the minimum wage,” Mr. Trumka said. “He’s changed the regulation to take overtime away from a couple of million people. He’s proposed trillion-dollar cuts to Medicare and Medicaid. He’s rolled back health and safety standards toward workers.”
As a crowd of about 200 waited for Ms. Warren’s arrival on a sunny day, blocking the entrance to the idled plant, some sang along to a recording of Aretha Franklin’s “Respect,” the message directed at G.M. management. Many wore red T-shirts that said “Solidarity, No More Bull.”
In many ways, Ms. Warren’s central message, which emphasizes the struggles of middle- and low-income Americans, appeals to the same frustrations that Mr. Trump tapped into in 2016 — pent-up anger at an economy and system that workers view as unfair.
“Personally for me, this is a lot bigger than G.M. and the autoworkers,” said Mike Mucci, a worker who was on the picket line outside the Detroit plant on Sunday. “This is a fight for the middle class.”
Mr. Mucci, 43, who said he was not a fan of Mr. Trump, predicted that the president would “do anything he could to try to take credit for saving the U.A.W.”
Robert Hatline, 61, a retired G.M. worker who voted for Mrs. Clinton in 2016, said he was impressed with Ms. Warren’s plan to provide free public college. “I like what she says if she can do it,” he said. “I want to see student loans reduced off these kids.”
Carla Duckett, 60, a team leader on the Cadillac line here, said she would support whichever Democrat the U.A.W. endorsed. “I just know we’ve got to support the Democrats this time,” she said.
Ms. Klobuchar, who delivered doughnuts when she visited the picket line on Thursday, described how G.M. workers had suffered during the economic downturn and how the company had “bounced back.”
“This particular company has been making a lot of money and that means to me that you got to share it with your workers,” she said. “And that means better wages, that means health care, and that also means the long-term benefits that our workers deserve. That’s what this is about. It’s about shared prosperity, that we’re all in it together.”
Mr. Sanders’s Wednesday visit will follow a stop on Tuesday at a labor rally in support of the Chicago Teachers Union.
In August, Mr. Sanders released his Workplace Democracy Plan, which he has said would double union membership during his term and give workers increased workplace protection.
Mr. Ryan, who cited the closing of the Chevrolet Cruz plant in Lordstown earlier this year as the impetus for his presidential campaign, joined G.M. workers outside the idled plant last Monday.
“When a group of workers lose their jobs, it causes massive disruption in the whole community,” Mr. Ryan tweeted. “Kids of these workers have to deal w/ either move to a new state & school; or the trauma of a parent being laid off. Educators are often left to help pick up the pieces.”
G.M. last year announced its plans to close the four plants, including the one in Lordstown, with hundreds of workers electing to relocate to other G.M. facilities. The Hamtramck plant, which makes the Chevrolet Impala and the Cadillac CT6, is slated to shut down in January.
The idled plants have become an issue in the negotiations, with workers also pushing G.M. to improve wages, add jobs at other facilities, pay a greater share of employee health care costs, and close or narrow the difference between pay rates for new hires and veteran workers.
Pointing to a $50 billion federal bailout of G.M. in 2009 following the financial crisis, as well as union concessions, many G.M. workers have said they view the decisions as particularly callous. The company paid the government back most of the money.
The Democratic presidential candidates have joined picket lines in previous strikes, including the McDonald’s strike in May and the Stop & Shop strike in April, as they jockey for union endorsements.
But with a couple of isolated exceptions — the United Electrical, Radio and Machine Workers of America endorsed Mr. Sanders last month, and the national firefighters’ union endorsed former Vice President Joseph R. Biden Jr. in April — unions have been holding back, many waiting until they see which Democratic candidate wins the support of the rank and file.
Maggie Astor, Neal E. Boudette, Katie Glueck and Noam Scheiber contributed reporting.