Three Democrats running for president marched with striking McDonald’s workers on Thursday — their latest effort to win over working-class voters on the picket lines.

Julián Castro, Washington Gov. Jay Inslee, and New York City Mayor Bill de Blasio rallied with workers in three of the 13 cities where McDonald’s employees had organized a work stoppage to demand a $15 hourly wage, and the right to join a labor union.

“We’re here to tell McDonald’s that it’s not acceptable to pay workers a wage they can’t live on,” Castro told a crowd of McDonald’s workers in Durham, North Carolina, early Thursday.

The former Housing and Urban Development secretary got loud cheers from the crowd when he said his campaign won’t spend money at McDonald’s until the company pays workers $15 an hour, recognizes their right to form a union, and addresses complaints of widespread sexual harassment and violence against workers.

It’s unclear exactly how many fast-food employees are striking, and whether restaurants were experiencing serious disruptions. At least some restaurants had to close their doors.

The multi-city strike was timed to send a message to McDonald’s executives as they meet for the company’s annual shareholder meeting Thursday in Dallas. Fast-food workers have been protesting for years as part of the Fight for $15 movement. For the most part, McDonald’s has ignored them. But the group, which is backed by the Service Employees International Union, has since become an influential voice in the growing labor movement, pushing lawmakers in seven states to adopt a $15 minimum wage.

Democrats vying for the White House know that union support is crucial to their success, and they’ve been working hard to win over fast-food cooks and cashiers. Picketing with striking workers is now a regular stop on the campaign trail for several 2020 contenders.

McDonald’s workers know how to wield their influence

In recent months, fast-food workers have seen their efforts to raise wages pay off. They’ve already pushed lawmakers in four states to adopt a $15 minimum wage so far this year, and they’re building support in Congress for a similar hike at the federal level.

Last month, the presidential frontrunners pledged their support for a $15 minimum federal wage. But despite their political successes so far, McDonald’s workers are frustrated that the company has done little to address their concerns.

This week, about two dozen women in 20 cities filed sexual harassment complaints against the company with the US Equal Employment Opportunity Commission. Most are new cases that must first go through an EEOC investigation, but a handful involve complaints women filed with the agency last year, which they are now taking to court. The group of current and former McDonald’s workers say the company has done nothing to keep female workers safe at its franchise and corporate-owned restaurants.

Workers also say they need a labor union to force McDonald’s to take action.

But the company, based in Chicago, wants to avoid that at all costs. It won’t recognize McDonald’s workers as its own employees because most of them work at independently-owned franchises.

The company is locked in a labor dispute over whether the fast-food chain is considered a “joint employer,” and is therefore partly liable for labor violations committed by individual franchises. McDonald’s argues that it is not a joint employer, so it cannot be held legally accountable for sexual harassment and other illegal workplace behavior at any of its independently owned restaurants.

However, workers disagree. They say the company has too much control over franchise restaurants and workers to make that claim.

Because McDonald’s doesn’t view cashiers and cooks as its employees, the company won’t get involved in setting hourly pay rates for restaurant workers, and won’t bargain with them through a labor union. So McDonald’s workers keep making as much noise as possible.

In Dallas, security guards blocked a group of workers who wanted to give a list of their demands to McDonald’s executives at the hotel where they were gathering. “We are just trying to earn a living wage like you,” said Bleu Rainer, a McDonald’s employee from Tampa, Florida.

Rainer and three other McDonald’s workers had just participated in a video town hall with Sen. Bernie Sanders (I-VT). “We need a seat at the table with McDonald’s to solve these problems,” one of them said to Sanders. “How would you use the power of the office to bring McDonald’s to the table to talk to workers like me?”

“Maybe we’ll invite the CEO into our office to sit and talk to you guys,” Sanders responded, adding that he would make it easier for everyone to join unions and would require corporations to reserve board seats for employees.

Fellow 2020 contender Bill de Blasio marched with McDonald’s employees in Des Moines, Iowa. Jay Inslee rallied with workers in Chicago.

And with a record number of US workers going on strike these days, we’re likely to see even more Democratic candidates on the picket line.

Candidates are courting voters at picket lines

Democrats have long focused their campaigns on working-class voters, particularly unionized workers, who are the most likely to go to the polls. But in the 2020 race so far, candidates are doing more than just attending town hall meetings. They’re actually joining the demonstrations, and actively supporting the workers.

Sens. Elizabeth Warren and Bernie Sanders, South Bend Mayor Pete Buttigieg, and former Vice President Joe Biden all made a point of showing their support for Stop and Shop supermarket employees during a 10-day strike against a corporate push to cut employee benefits.

“When workers fight, workers win,” Warren said to a group of workers picketing outside one of the chain’s stores in Massachusetts. She even brought them doughnuts.

But the labor movement has changed a lot in the past 20 years, and it poses some interesting challenges for Democrats, as Vox’s Tara Golshan explains:

The tensions within the labor movement in America are representative of the larger pulls within the Democratic Party. While Joe Biden is trying to shore up the old coalition of white working-class voters that defected to Trump, candidates like Sens. Warren, Kamala Harris, and Sanders, too, are attempting to speak to an increasingly progressive working class that didn’t see a champion in either Trump or Clinton in 2016.

While union membership overall has been decreasing, people of color are joining unions in higher numbers. Groups like Service Employees International Union, which mainly represent women working in industries like home care, housekeeping, and health care, say their members didn’t vote for Trump. They stayed home.

The McDonald’s strike reflects the changing demographics of the labor movement. The American fast-food workforce may have no history of unionizing, but the workers marching on Thursday mobilized thousands of workers regardless. Most of them were black and Latina women. And 2020 candidates clearly see this as an opportunity to gain more support.

As the primary elections get closer, we’ll likely see more of the 20-plus contenders in the Democratic field on the picket line.