“You can’t change a corrupt system by taking its money,” Sanders said in a statement. “That is why I am calling on every Democratic candidate in this election to join us in rejecting money from the insurance and drug industries.”

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Candidates who are unwilling to do so, Sanders said, “should explain to the American people why those interests believe their campaigns are a good investment.”

Sanders’s pledge underscores his desire to expand the debate over health care beyond the competing policy proposals and shows how his campaign is seizing on health care as an effective issue to wield against the more centrist Biden — a fight Biden appears to welcome. Sanders has opted not to hold fundraisers catering to wealthy donors and is not seen by insurance and drug companies as an ally.

Biden, in contrast, has some ties to those industries. He has received four-figure donations from executives at companies such as Merck & Co. Inc., Independence Blue Cross and Gilead Sciences, campaign finance records show.

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Other Democratic candidates have also accepted contributions from industry officials or lobbyists. According to campaign finance records, they include two candidates on the rise in the polls: Sen. Kamala D. Harris (Calif.) and South Bend, Ind., Mayor Pete Buttigieg.

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Candidates who take such corporate donations dismiss the notion that it makes them beholden to powerful industries, pointing to their record and their platform. Sen. Elizabeth Warren (D-Mass.), perhaps Sanders’s most direct rival for the party’s liberal voters, has eschewed big-donor fundraising events.

Sanders’s pledge marks one of his most aggressive moves of the campaign. Since launching a second run for president in February, Sanders has mostly focused on his own pitch for transformational change.

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But he has lost ground to Biden and other candidates in recent polls and fundraising, creating pressure to find ways to regain momentum.

The Sanders campaign launched a page on its campaign website listing dozens of companies whose donations would not be accepted under his new pledge. His challenge says that candidates will not accept contributions over $200 from lobbyists or executives working for those companies. It also applies to political committees representing such businesses.

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But the pact “does not apply to rank-and-file workers employed by pharmaceutical and insurance companies,” the website says.

In his speech, Sanders will “confront the Democratic opponents of Medicare-for-all,” according to his campaign. The senator has long pushed for a single-payer health care program, but as the idea gains attention and support in the Democratic Party it is coming under more scrutiny than ever before, drawing criticism from members of both parties.

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Campaigning in Iowa on Tuesday, Biden called Medicare-for-all a “noble idea,” but he warned that it would remove the insurance choices Americans have under current law. He has also warned that a national transition to an entirely new health-care system could create dangerous instability. Biden unveiled a plan this week that would expand the Affordable Care Act, President Barack Obama’s signature health law, with an optional public health insurance program.

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Sanders and his top aides have aggressively countered Biden, arguing that the former vice president’s proposal is outdated and does not go far enough to ensure that Americans have affordable and complete coverage. They have presented Medicare-for-all as the only way to fully accomplish that, noting that virtually every other industrialized country has a version of that system.

At a recent campaign stop in Iowa, Sanders dedicated some time in his speech to addressing some oft-stated worries about his plan. His aides say they are placing a growing emphasis on defending it against detractors.

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“There’s been a lot of misinformation out there about Medicare-for-all and what it means for the average person,” said Jeff Weaver, a senior Sanders campaign adviser.

In the midterm elections, Democrats campaigned successfully on health care, criticizing Republicans for trying to repeal the ACA and highlighting their efforts to protect the law, which has become more popular over time.

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Some Democrats worry that talk about scrapping the ACA in favor of a new system, and spurring a debate about largely ending private insurance, could cede that political advantage to Republicans. Trump and his allies have seized on the Democratic push to remake the health-care system, seeking to turn the tables by casting them as extremists looking to force people into an undesirable plan.

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“I think it could be a political problem,” said Democratic Montana Gov. Steve Bullock, who is seeking the party’s nomination and favors a public option. “If Barack Obama got beat up for saying, ‘If you like your health plan or you like your doctor you can keep it,’ well, who knows what would happen in this instance.”