Bernie Sanders, the self-described political revolutionary who has struggled to expand his support base in a crowded Democratic presidential field, offered his most comprehensive critique yet of rival Joe Biden on Tuesday — a reflection of the candidates’ increasing eagerness to challenge the primacy of the former vice president.

The senator from Vermont, in an interview with The Washington Post, criticized Biden on his health-care plan, his foreign policy record and his ability to win crucial voters in the Upper Midwest states that were carried in 2016 by President Trump, calling Biden an enabler of the “deregulation of Wall Street” that led to “incredible pain” for many Americans.

“The differences between Joe and me on foreign policy, on domestic policy, is pretty significant,” Sanders said of the former vice president. “More importantly, our vision for the future of this country is very different.”

Biden, who leads in the polls, has increasingly become a target of his Democratic competitors in recent weeks, facing rebukes over his record on race from Sen. Kamala D. Harris (D-Calif.) and now a sustained offensive from Sanders. Biden has responded with tougher jabs at his challengers after initially sidestepping them to focus on Trump.

Tuesday’s interview, broadcast online as part of The Washington Post Live’s candidate series, signaled that Sanders sees an aggressive fight with Biden over issues the senator has long championed as the way to boost his campaign, allowing him to portray Biden as part of a problematic establishment and himself as an agent of urgently needed change.

It comes at a precarious moment for the Sanders campaign, as the senator has shown signs of struggling to adapt to a fast-changing race in which others have seized momentum and Biden continues to hold onto his lead. Sanders essentially finds himself competing with Sen. Elizabeth Warren (D-Mass.) and Harris for the role of chief liberal challenger to Biden.

His critique of Biden on Tuesday stood out for advancing not just a policy argument but a political one as well. Sanders aides and allies say they embrace opportunities to clash with Biden, wagering that will attract working-class voters who are torn between the two candidates.

“To beat Trump, we need a candidate who is going to run a campaign of energy and excitement,” Sanders said.

Although Sanders was an early critic of Biden after the former vice president launched his campaign, the two were not locked in a prolonged dispute until now. And both sides appear eager for the fight, unlike Biden’s conflict with Harris, which was driven by her attacks on his record on busing and his comments on working with segregationist senators.

Tensions between Sanders and Biden have been rising for several days over health care. Biden released a plan this week that would expand the Affordable Care Act with an optional public insurance program, and he warned that shifting to a more sweeping Medicare-for-all initiative, as Sanders advocates, is risky and could hurt patients.

During a Tuesday speech in Manning, Iowa, Biden attacked the plan Sanders is pushing, calling it a “noble idea” but one that would remove people’s insurance choices.

“My administration will drive common-sense reforms to make it easier, not harder, for the American people living in rural America to receive high-quality care,” Biden said.

Sanders responded to Biden’s critique by saying, “Of course he’s wrong,” calling the criticism of Medicare-for-all “absurd” and attacking Biden’s proposal as unequal to the health-care crisis.

“Times change, and we have got to go further,” Sanders said.

Sanders is scheduled to deliver a speech in Washington on Wednesday defending his plan for a government-run health system. Medicare-for-all, a version of what’s often called a single-payer system, has long been one of Sanders’s signature policy proposals.

Biden’s campaign says his plan would cost about $750 billion over 10 years. Asked what his own plan would cost, Sanders said it would be somewhere between $30 trillion and $40 trillion over 10 years.

He stressed, however, that it would offer huge savings in health care, even if it would result in higher taxes for middle-class Americans.

Sanders also sought to sharpen the contrast between himself and Biden on foreign policy and trade, noting the former vice president’s vote authorizing the Iraq War and support for far-reaching trade deals.

“How is that going to play in the Midwest, which was decimated — in Michigan, Wisconsin, other states which were decimated — by these terrible trade agreements?” asked Sanders. “Do I think that the workers in those states are going to feel very kindly to a guy who supported those agreements?”

In another indication of its intention to target Biden, the Sanders campaign launched a page on its website allowing people to guess whether various quotes were uttered by Biden, Trump, Senate Majority Leader Mitch McConnell (R-Ky.) or an insurance executive — pointedly lumping in Biden with a series of Democratic villains.

Sanders’s second run for president got off to a fast start in February and March, drawing big crowds, strong polling numbers and gobs of cash for his campaign coffers.

But recent surveys show that his once-clear grip on second place has disappeared; one poll showed him slipping to fifth place in New Hampshire, a state he won decisively over Hillary Clinton in the 2016 primary. Sanders on Tuesday dismissed that poll, conducted by Saint Anselm College, as an outlier. Another poll, from CNN and the University of New Hampshire, shows him running second in the state.

Sanders on Tuesday also took aim at Trump, criticizing his racist remarks about four members of Congress and accusing the president of making a calculated decision to sow division to benefit his reelection bid.

“Trump may be crazy, but he’s not stupid,” Sanders said. “I think what he believes is it is a good strategy to divide people up.”

Sanders sought to underscore his ability to defeat Trump, though he conceded that Biden also runs strongly in head-to-head polls against the president. When it comes to Trump’s rhetoric, Sanders repeated his assertion that Trump has used racist language like no other president in modern history.

“Can anyone imagine a conservative president like [George W.] Bush bringing forth these racist comments?” Sanders asked.

At times in Tuesday’s interview, Sanders was brusque. At other moments, he was jocular.

Some voters have said Sanders’s age — 77 — makes him less appealing to them as a candidate. He took on this critique, saying age is only one of many factors voters ought to consider.

He said he is in good health and could not remember the last time he missed work because of illness, and he jokingly challenged Trump to a mile-long run. When asked about Biden’s comment Tuesday on MSNBC’s “Morning Joe” challenging Trump to do push-ups onstage, Sanders said, to laughter, “I have no comment on that.”

The senator has campaigned much as he did in 2016: railing against foes in both parties, playing up his independent streak, and focusing on such ideas as increasing the minimum wage, providing universal health care and reining in the power of wealthy corporations.

During the interview, Sanders was critical of large companies, saying that Amazon “is moving very rapidly to be a monopoly.” If he were elected, Sanders said, his Justice Department would move to break up big companies like Amazon and Facebook. (Amazon founder and chief executive Jeff Bezos owns The Washington Post.)

In a crowded field with historic diversity and fresher faces, Sanders’s familiar strategies have produced uneven results.

After leading the Democratic pack in fundraising during the first quarter of the year, Sanders trailed three competitors in direct campaign donations in the second quarter.

Sanders advisers argue that he can deliver broad change like no other candidate, and they emphasize that he has long been a standard-bearer for ideas that other contenders have touted only recently.

Much as he did in 2016, Sanders has often cast himself as a target of the political establishment.

“I don’t say it’s aligned against me in a paranoid way,” he said Tuesday, adding, “We are taking on Wall Street, we are taking on the insurance companies.”

Cleve R. Wootson Jr. in Manning, Iowa, contributed to this report.