PHILADELPHIA — Former vice president Joe Biden and Sen. Bernie Sanders (I-Vt.) clashed sharply over health care in separate appearances before union members on Tuesday, intensifying one of the central policy disputes in the Democratic presidential race.

Speaking at a forum hosted by the Philadelphia Council of the AFL-CIO, Biden touted his plan to expand the Affordable Care Act with an optional public insurance program. Without naming Sanders and Sen. Elizabeth Warren (D-Mass.), he eagerly criticized the competing proposal they have championed as injurious to organized labor.

“I have a significant health care plan. But guess what? Under mine, you can keep your health insurance you’ve bargained for if you like it,” Biden said. “If you don’t, you can move it, and you can buy into a public plan.”

Sanders promoted his Medicare-for-all proposal, under which the government would be the sole insurer for all Americans. And he highlighted an ongoing labor dispute in which health care costs have been shifted onto the union backing striking workers.

The collision showed how heavily health care is factoring into the crucial fall stretch of the race, which polls show has at least temporarily become a three-way competition between Biden, Warren and Sanders.

In delivering their remarks before a union audience, Biden and Sanders took their cases directly to the working class voters expected to play a significant role in the nomination contest.

The former vice president told the crowd, “You’ve broken your neck to get” your health-care plans and “you’ve given up wages to get it.” He added, “You should be entitled to keep it. And no plan should take it away from you if that’s what you decide.”

Biden’s remarks came on the same day that a standoff between the United Auto Workers and General Motors escalated, with GM shifting health-care costs onto striking workers. Later, when Sanders spoke, he brought this up.

“That’s the kind of ugliness and greed that we are seeing every day,” Sanders said, prompting boos about GM’s conduct from the crowd. Sanders, who recently changed how his Medicare-for-all plan would protect union workers — giving them more leverage amid concerns from some in organized labor about his plan — defended it as the best path to achieve complete coverage for Americans.

Addressing reporters afterward, Sanders argued that under his plan, the situation with GM would be averted.

“Here you have the situation where the UAW is now on strike, 49,000 workers. I’m sure that in that 49,000, there are family members who are seriously ill,” he said. “Under Medicare-for-all, whether you’re working, whether you’re not working, whether you go from one job to another job, it’s right there with you.”

Sara Nelson, the influential president of the Association of Flight Attendants, an AFL-CIO union, appeared to warn Biden over his comments on Twitter. “A note to anyone who wants to use union members as a wedge to oppose #MedicareForAll: @UAW has one of the best plans in the country, but management can still use it to hold workers hostage. #M4A puts power back in our hands,” she wrote.

Several of the unions represented at the forum also belonged to affiliates that have endorsed some form of single-payer health care. Blossom Kaleo, 41, a part-team teacher and member of the Philadelphia Federation of Teachers, said that it was wrong to say that union members were afraid to replace their current coverage with a universal plan. The American Federation of Teachers has supported the Sanders bill.

“I can’t even afford what our union offers,” Kaleo said. “I’m for Medicare-for-all because it’s a human right.”

Warren, who did not attend Tuesday’s forum, strongly supports Medicare-for-all. “I’m with Bernie,” she said at a June debate. Biden seized on her comment in last Thursday’s debate.

“I know that the senator says she’s for Bernie, well, I’m for Barack,” said Biden, who has frequently invoked the 44th president and his service as Barack Obama’s vice president.

Concluding his remarks at the end of a question-and-answer session Tuesday, Biden argued that it’s not necessary to remove existing laws to make progress, emphasizing one of the core themes of his campaign.

“You don’t have to do it by taking down and ripping away everything that’s been in place before. That’s not necessary,” Biden said.

The Democratic candidates are making more explicit appeals to union workers than in past campaigns, as the focus on working class voters has sharpened in the party after President Trump in 2016 benefited from the support of many of them.

Democratic activists felt the party did not speak to union workers’ economic and social concerns, especially in upper Midwestern states where Trump was victorious. Attempts to correct that have been evident in many of the platforms the candidates have embraced.

Biden and Sanders, the only candidates who have won endorsements from national unions this year, have sought to portray themselves as champions of workers. Warren, who on Monday won the endorsement of the Working Families Party, also has underscored support for measures benefiting working class Americans.

But most large unions are in no rush to pick sides in the crowded and fluid race. For the moment, there does not appear to be a consensus candidate in the labor community. Four years ago was different, as organized groups largely lined up behind Hillary Clinton.

For Sanders, the appearance here marked his first campaign stop since Sunday. With a voice that was noticeably raspy at last Thursday’s debate, Sanders canceled most of his Monday and Tuesday schedule to rest his vocal cords, his campaign said.

His voice still a bit hoarse as he spoke, Sanders received some of the loudest cheers of the day. But he also got some heckles from the back of the room.

The Vermont senator returned to the trail facing some new challenges and turmoil. Warren’s Working Families Party endorsement was a blow to Sanders, who won the group’s support in the 2016 campaign.

Sanders also shook up his campaign in New Hampshire, a state where he defeated Clinton by a wide margin in 2016 but faces a more competitive race this time. His campaign replaced its New Hampshire state director, Joe Caiazzo, with Shannon Jackson, who managed Sanders’s 2018 Senate reelection effort. The campaign announced the move to supporters on Sunday.

Tuesday’s event was the first presidential summit hosted by the Philadelphia Council AFL-CIO, which includes over 100 local unions that represent nearly 200,000 workers. In addition to Biden and Sanders, New York Mayor Bill de Blasio, tech entrepreneur Andrew Yang, Sen. Amy Klobuchar (D-Minn.) and billionaire activist Tom Steyer also fielded questions here.

The AFL-CIO, an influential coalition of 55 national and international unions representing 12.5 million workers, is taking time before deciding whether to endorse a candidate. It has scheduled a candidate forum for next spring, after which it might pick a favorite, according to a spokesman.

An endorsement by the general board of the AFL-CIO will require a two-thirds vote of membership, according to the spokesman. The group endorsed Clinton in June 2016.

Both Biden and Sanders sought to make a personal pitch to union members in the crowd. Biden talked about growing up in a working class community in which many of the people around him went on to become union workers. Sanders told the audience he came from a “working class family who lived paycheck to paycheck.”

In his turn onstage, Sanders also took aim at opponents such as Biden who voted for the Iraq War and sweeping trade deals.

“Not only did I vote against NAFTA and [permanent trade relations] with China, I helped lead the opposition to them. Joe voted for both of those agreements,” Sanders told reporters afterward. “I did not vote to bail out the crooks on Wall Street. Joe did.”

In his remarks, Biden sought to cast his long career in government and politics — something critics seeking younger leadership have pounced on — into a positive.

“The bad news is I’ve been around a long time. The good news is I’ve been around a long time. You all know me,” he said.