ALTOONA, Iowa — Senator Bernie Sanders of Vermont on Wednesday rolled out an ambitious plan to strengthen organized labor in the United States, setting a goal of doubling union membership in his first term as president.
Mr. Sanders was among the biggest names in a procession of Democratic presidential candidates who affirmed their support for unions on Wednesday in speeches at a labor convention near Des Moines.
“If there is going to be class warfare in this country, it’s time that the working class of this country won that war and not just the corporate elite,” Mr. Sanders said.
More than a dozen candidates spoke on Wednesday at the convention, held by the Iowa Federation of Labor, A.F.L.-C.I.O. For the Democrats, it was another chance to advance their argument that the economy is failing working people — and that strengthening unions is a critical step to empowering workers.
Senator Amy Klobuchar of Minnesota spoke of her grandfather, a union iron ore miner. Former Representative John Delaney of Maryland told the crowd of his father, a union electrician. Senator Elizabeth Warren of Massachusetts said she had helped graduate students, casino workers and airport workers unionize.
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Senator Michael Bennet of Colorado rolled out a new policy plan focused on American workers, particularly those without a four-year college degree. “In this election, it won’t be enough to run against Donald Trump’s lies and empty promises,” Mr. Bennet said. “We need to make crystal clear that the Democratic Party stands for working people.”
The candidates’ speeches also put a spotlight on the continuing divisions over whether to move to a “Medicare for all” system that would eliminate private health insurance — and the ramifications that would have for union members, whose leaders often negotiate generous health care plans with employers at the expense of raising wages or securing other benefits.
Several candidates offered warnings about getting rid of private health insurance — a move supported by Mr. Sanders and Ms. Warren under Medicare for all. Former Vice President Joseph R. Biden Jr., who wants to allow people to keep their private insurance but give them the option of signing up for a government-run health plan, was among those to raise the issue.
“You’ve negotiated really hard for your benefits with your union with the employer,” he said. “And my plan, you get to keep it — you don’t have to give it up.”
Mr. Sanders’s labor plan appeared designed to pre-empt that criticism. It would require that employers dedicate the savings they receive when they no longer provide union-negotiated health plans toward raising wages and adding other benefits.
The candidates appeared at the convention amid increasing concerns that the United States is at risk of slipping into a recession. President Trump said on Tuesday that he was considering “various tax reductions,” including a payroll tax cut, to stimulate the economy, though he backed off those comments on Wednesday.
The candidates at the event on Wednesday generally stayed away from talk of a recession but used the opportunity to emphasize their support for organized labor.
Senator Cory Booker of New Jersey said, “The American dream is not possible without unions.” Mr. Biden said the “only reason we have a middle class is unions.” Mayor Pete Buttigieg of South Bend, Ind., said that “freedom requires the freedom to organize for a good day’s pay for a good day’s work.” Julián Castro, the former housing secretary, promised to appoint a labor secretary “who will champion the ability of labor unions to organize.”
Mr. Sanders called his new labor plan “the strongest pro-union platform in the history of American politics.” It would allow a majority of workers to form a union simply by signing authorization cards, rather than winning a secret ballot election, which would make unionizing substantially easier.
It would also end so-called right-to-work laws that exist in more than half the states, which allow workers to opt out of paying union fees if they choose not to join a union, even if the union bargains for their contract. Unions argue that this allows workers to receive the benefits of membership without helping to offset the costs of providing such services.
Mr. Sanders would also create a system in which workers bargained for wages and benefits by industry, rather than with particular companies, as federal labor law currently requires. That would make it possible to increase wages and benefits for more workers at once, and potentially ease employers’ concerns that raising wages would put them at a competitive disadvantage.
And Mr. Sanders would require employers receiving federal contracts to pay workers at least $15 per hour and to remain neutral in union organizing campaigns.
The plan “is an important recognition of the fact that tinkering around the edges isn’t going to be enough to return power to American workers in our economy,” said Sharon Block, a former National Labor Relations Board member appointed by Mr. Obama, who is executive director of the Labor and Worklife Program at Harvard Law School.
Also on Wednesday, the president of the Service Employees International Union, one of the largest and most politically powerful unions in the country, laid out a plan for changing federal labor laws that overlapped with Mr. Sanders’s proposal.
In a speech in Milwaukee, the S.E.I.U. president, Mary Kay Henry, emphasized the need for industrywide bargaining of the sort Mr. Sanders proposed, and said that candidates who declined to support the union’s plan “can’t count on our support.”
Ms. Henry said in an interview that Mr. Sanders had answered “the demands he’s been hearing from fast food workers,” which the union has been working to organize.
The Sanders campaign worked closely with the S.E.I.U. and other unions on the plan before rolling it out, according to a campaign aide.
Making it easier for workers to unionize would generally benefit Democratic candidates, according to both supporters and opponents of such proposals. Union members often play a key role in canvassing voters, and members tend to vote for Democrats at higher rates than other voters with similar demographic characteristics.
“Research indicates that unions can shape the ways that their members think about politics,” said Alexander Hertel-Fernandez, a political scientist at Columbia University who studies the politics of labor.
A handful of other candidates, including Mr. Buttigieg, Mayor Bill de Blasio of New York and Gov. Jay Inslee of Washington, have also proposed significant overhauls of labor law.
Ms. Henry issued a warning on Wednesday to candidates who had not put forth such plans that the union’s support might not be forthcoming. “It’ll be part of what we evaluate,” she said.
Joe Trippi, who helped broker the S.E.I.U.’s endorsement of Howard Dean in 2003, when he managed the former Vermont governor’s presidential campaign, said he doubted that the threat would force the eventual Democratic nominee to adopt the service employees’ labor agenda if he or she had not done so already.
“I think it’s more likely this is a marker to get candidates to move toward them prior to Iowa and the primary season,” Mr. Trippi said in an email. “By the time someone is the nominee it’s often too late to move them. Putting a marker down now is smart.”
Thomas Kaplan reported from Altoona, Iowa, and Noam Scheiber from New York.