Four 2020 Democratic presidential candidates will visit striking McDonald's workers in the coming days as a jammed primary field tries to prove its chops with organized labor.

Sen. Kamala Harris, D-Calif., heads to Las Vegas on Friday to join striking fast food workers in a state that will hold the third Democratic nominating contest next year. On Saturday, South Bend Mayor Pete Buttigieg, former Rep. Beto O'Rourke and Sen. Cory Booker, D-N.J., will visit laborers' protests in Charleston, South Carolina. The state is also important in the 2020 primary season: it will vote fourth.

McDonald's workers — who helped to start the Fight for $15 movement that has contributed to minimum wage hikes around the country — have pushed for years for union membership and a $15 per hour pay floor. Employees have also urged the chain to better handle accusations of sexual harassment and violence at its locations. They have increased pressure this year to force the company to make changes.

"My main motivation is we're fighting for $15 an hour because right now we're just living off of poverty wages and in Vegas the cost of living is just continuing to go up," Martin Macias said in Spanish, according to a Fight for $15 translator who declined to be named. Macias has worked at a Las Vegas McDonald's for five years and will strike for the first time Friday as part of the Fight for $15 rally.

For Democrats, the strikes offer a platform to court key working-class voters in a party traditionally backed by organized labor. As two dozen candidates jockey for support in a Democratic Party increasingly focused on reining in huge corporations, they have jumped at every chance to vouch for workers.

The vast majority of the Democratic presidential field has at least sent messages of support for McDonald's employees during strikes earlier this year. Multiple candidates have rallied with the chain's workers. Most recently, Sen. Bernie Sanders, I-Vt., marched with McDonald's laborers in the first in the nation caucus state of Iowa.

It's not just McDonald's. Candidates have made campaign stops at Walmart's annual meeting and joined striking Stop & Shop workers to make the case for higher wages and better representation for workers.

Macias, through the translator, said Harris' presence on Friday is important for workers, but he stressed that he wants her to continue to vouch for fast food workers after she leaves Las Vegas. He said the 2020 Democratic candidates "know that they need the vote of the masses" in the 2020 primary, according to the translator.

Democratic contenders hope in part to win the support of the country's biggest labor unions, which have so far withheld endorsements early in the 2020 contest. Only a couple notable unions have made their intentions clear. The International Association of Firefighters backed former Vice President Joe Biden, while the New York Hotel and Motel Trades Council supported New York City Mayor Bill de Blasio.

In a statement to CNBC, Harris said, "I'm proud to stand in solidarity with the working women and men fighting for the wages and benefits they deserve here in Nevada." She added that unions "are under attack from the Supreme Court to the White House," saying "we need a leader and president who will stand with unions and working people."

Also in a statement, Buttigieg said "these workers are living with low wages, harassment, frequent violent assault — and they deserve better." He called it "critically important" for presidential candidates to bring attention to labor issues during a closely watched campaign.

Candidates have used the McDonald's strikes not only to highlight wage hikes and union formation, but also to make their cases on related issues. The worker complaints against the fast food chain come amid a broader national reckoning — and political focus — on workplace harassment.

O'Rourke's campaign also tied the strike to efforts to boost people of color in South Carolina — a state where black voters accounted for about 60% of the 2016 Democratic primary vote, according to The New York Times.

"Beto recognizes that our economy isn't working for too many Americans – particularly those in communities of color. We need to confront that inequality today by ensuring that every workplace is free of sexual harassment and violence and that every worker can earn enough to support themselves and their families," the campaign said in a statement.

The Chicago-based McDonald's employs about 850,000 people across its American locations. Franchisees own more than 90% of those locations. Those store owners set wages, not the corporation.

The fast food behemoth has faced consistent pressure over wages and benefits. Despite repeated strikes and lobbying, efforts by McDonald's workers to unionize and secure a $15 per hour minimum wage have fallen short.

In addition, the Fight for $15, the Time's Up Legal Defense Fund and the American Civil Liberties Union filed 23 new harassment complaints against McDonald's last month. They also argued the company has not done enough to address harassment against women working at the company.

McDonald's says it has taken steps to address concerns. Joe Jasper, officer at the National Franchise Leadership Alliance — which represents more than 2,000 McDonald's franchisees in the U.S. — said "no level of harassment has a place inside a McDonald's."

"We've also sought to do much more than listen," he said. "Through our collective actions, and through tools we have been rolling out, we're working hard to provide all of our employees the support and resources they need to work in a safe environment that fosters opportunities and trust."

Meanwhile, McDonald's CEO Steve Easterbrook previously said the company has taken steps to improve its handling of harassment, abuse and retaliation, according to a letter viewed by CNBC.

The company has also tried to show efforts to respond to pressure over wages. In a March letter to the National Restaurant Association, McDonald's Vice President of U.S. Government Relations Gemma Gent said the company would no longer lobby against minimum wage increases at the local, state and federal level.

This year alone, four states have approved a $15 per hour minimum wage. House Democrats also hope to pass a bill to raise the wage floor to $15 nationwide, though the plan is unlikely to get through the GOP-held Senate.

In March, Gent wrote that the average starting pay at company-owned stores was more than $10 per hour, according to The New York Times. She said wages at franchisee-owned stores were "likely similar." That hourly pay tops the federal minimum wage.

Macias, for one, does not think McDonald's has done enough to address workers' concerns.

"While they're making money, which is their priority, they're not really showing an interest for their employees," he said through the translator.

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