The workers were chanting “Shut it down!” when photographers turned to a figure approaching from the end of the block, a Dunkin’ donuts box in her hands.

Sen. Elizabeth Warren (D-Mass.) had shown up at a picket line manned by striking Stop & Shop workers in Somerville, Mass., during the 11-day strike at the grocery chain last April.

“These giant companies think they can knock unions back,” Warren told the crowd through a bullhorn. “They think they can push us back. But what they need to understand is that unions are here to stay!”

A few days later, former vice president Joe Biden was at another Stop & Shop picket line, excoriating Wall Street bankers and CEOs. “You built America!” he boomed to the crowd.

The next day, Sen. Amy Klobuchar (D-Minn.) and South Bend, Ind., Mayor Pete Buttigieg met with the striking grocers. Other presidential candidates, such as Sen. Bernie Sanders (I-Vt.), voiced their support on social media.

Barely a week old at the time, the strike of 31,000 workers at about 240 stores had suddenly found itself at the white-hot center of the 2020 campaign race.

The road to the presidential nomination next year is sure to be full of unforeseen twists and potholes as a crowded field of Democratic contenders dukes it out in a volatile political climate. But about a year into their race, one thing is clear: It leads through a thicket of striking workers, in a number of states, whether they are in front of a grocery store, an automotive factory or an elementary school.

This push comes as they try to dislodge some of the support President Trump has found in states that have lost tens of thousands of union jobs in recent years, including Wisconsin, Michigan and Pennsylvania. Hillary Clinton beat Trump in union households by only nine percentage points in 2016 — half of the 18-point margin Barack Obama won over Mitt Romney and well below the 30-point advantage Bill Clinton had in 1992, according to exit poll data. Questions remain about how those hundreds of thousands of votes potentially affected the race.

Democratic candidates are telegraphing their support for workers in a variety of ways. Warren launched her campaign in Lawrence, Mass., in front of a mill that was made famous by a strike led by a group of female workers in 1912.

“I am a union man,” Biden said during his announcement a few months later. Nearly all of the candidates have dramatically pro-union and pro-worker policy proposals in their platforms.

Political observers said the rush by 2020 hopefuls to embrace striking workers marks a new chapter, although unions have been nominally aligned with Democratic politicians on and off for years.

“Democrats have held their distance in several decades,” said Erik Loomis, a labor historian at the University of Rhode Island and the author of “A History of America in Ten Strikes.” “Now, going on a picket line is almost a requirement to be considered a serious candidate for the Democratic nomination. That’s basically unprecedented in American history.”

During the six-week strike that shut down production at General Motors, workers were greeted by Sanders, Klobuchar, former congressman Beto O’Rourke (D-Tex.), Sen. Kamala D. Harris (D-Calif.), Sen. Cory Booker (D-N.J.), Biden and Warren, as well as Rep. Tim Ryan (D-Ohio), who has since dropped out of the race. The parade of candidates was so dizzying that some workers at the Detroit-Hamtramck plant, which Klobuchar, Sanders and Warren visited — twice — said they had lost track of which ones had come by.

“They did earn some points,” said Hamtramck worker Chris Viola, 36, adding that Warren came out to stand with picketing workers during a downpour. “People are realizing that we’re out here and we want to be heard.”

The candidates’ visits have added to a sense of momentum in the world of labor organizing, which has seen the number of striking workers rise to the highest level in more than 30 years. Public support for unions, according to Gallup polls, is approaching a 50-year high. And the high-profile attention, which draws extensive media coverage, has helped turn the focus on the plight of workers and bring it into the center of the national political discussion.

Some have viewed the candidates’ visits with skepticism.

Jane McAlevey, a former organizer and a policy fellow at the University of California at Berkeley, argued in an essay she wrote for the Nation that candidates were offering workers photo opportunities but not actual power.

“The GM strike could have been an incredible opportunity for Democrats to drive home a core message: Trump promised workers not one plant would close on his watch, and now that promise is broken,” she wrote in the magazine. “The Democrats essentially ignored the chance.”

Vanessa Banks, the president of the United Auto Workers Local 1590 chapter, which represents workers at a General Motors plant in Martinsburg, W.Va., said she thought of the visits as “just politics.”

“They’re not helping us in any way,” she said.

But many union officials and workers said they appreciated the support.

“There really is an understanding by this crop of candidates that if you want to face wealth inequality and create good jobs, unions have to be a part of that, and not just say, ‘Unions matter,’” said Erikka Knuti, communications director for the United Food and Commercial Workers Union, which represents Stop & Shop workers. “We were very cognizant of not turning ourselves into a photo op for politicians, and I think that the folks that came did a good job.”

There are other political considerations at play, too.

Despite the fact that union participation in the United States has been declining for decades, unions still have effective “get out the vote” operations.

Joseph A. McCartin, a professor at Georgetown University, cited data from the Bureau of Labor Statistics that showed that Wisconsin lost 175,000 union members between 2008 and 2016; Michigan lost 165,000, and Pennsylvania lost 165,000.

“So key states that Trump ended up winning were states where unions got hammered,” he said in an interview. “[Democrats] saw how badly those losses hurt them in those states, and they saw how [former governor] Scott Walker’s impact on Wisconsin made Wisconsin available to Trump.”

Then there is the issue of endorsements. A majority of unions have yet to weigh in on the race, cautious perhaps because of the large pool of candidates.

“I think unions are really kicking the tires on a whole host of issues,” said Scott Treibitz, a political consultant who works with unions in Washington. “They know Bernie, they know Elizabeth, they know Joe, they know Harris and Booker, but they’re trying to see how they all react.”

Treibitz works with the International Association of Fire Fighters, which announced its endorsement of Biden in April.

The Democratic candidates have, by and large, released detailed plans to radically strengthen the rights of both unions and unaffiliated workers.

Sanders wants to give federal workers the right to strike, ban “at will” employment ⁠ — which allows companies to fire workers without cause ⁠ — and double union membership in the United States. Warren has called for banning the permanent replacement of striking workers and strengthening the National Labor Relations Board, which enforces laws meant to protect unions and organizing in workplaces.

Both of them, along with Booker, Buttigieg, Harris and Julián Castro, say they want to ban “right to work” laws that Republicans have championed in states throughout the country to allow workers to opt out of paying union dues.

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