Former vice president Joe Biden speaks at a campaign stop Saturday in New Hampshire. (Robert F. Bukaty/AP)

The fight over health care has burst to the forefront of the Democratic presidential race, with defenders of the Affordable Care Act, such as Joe Biden, and advocates of a broader Medicare-for-all system, notably Bernie Sanders, launching more aggressive attacks on each other.

The dispute, which has reverberated across the sprawling field, is exemplified by the increasingly hostile dynamic between Biden and Sanders. The former vice president released a proposal Monday to expand the ACA and warned that it would be dangerous to scrap it and enact Medicare-for-all. Sanders and his top aides rebutted those comments, and the senator from Vermont plans to deliver a speech Wednesday confronting critics of his proposal.

Biden, pitching his plan to a crowd in Iowa on Monday, warned that transitioning hundreds of millions of people to an entirely new Medicare-for-all plan would be “totally risky,” raising the possibility that coverage could lapse for some people, at least temporarily.

“Every second counts,” he said at a presidential candidate forum held by AARP. He argued that his plan offers a range of choices: “You can stay in your employer-based plan or you can move on. It’s the quickest, and it’s the most reasonable, rational, the best way to do it.”

Campaigning near a Philadelphia hospital set to be closed, Sanders called the current system “dysfunctional, because its goal is not to provide health care to all people as a right, but is to make as much money as possible for the insurance companies.”

And one of his closest aides issued a sharp attack on Biden’s plan. “What Vice President Biden is proposing at one time in history might have been a big step forward, but you can’t be Rip Van Winkle and wake up this year and say what was cutting edge 12 years ago is cutting edge today,” said Jeff Weaver, a senior Sanders adviser.

Perhaps no other issue has divided Democrats as sharply and as passionately as health care. Biden and Sanders, who in many ways represent the ideological poles of the Democratic field, are eagerly seizing the chance to clash with each other.

The health-care debate reflects a broader battle unfolding in the Democratic field that goes to the heart of the party’s future: How should Democrats balance idealism and pragmatism, and how far left should they move?

In many policy areas, Biden is the only leading candidate taking the more centrist approach, while Sanders, along with Sens. Elizabeth Warren (Mass.) and Kamala D. Harris (Calif.), embrace the more liberal view.

Disagreements about health care, an issue that polls show is critical to many voters, loom heavily over the general election campaign as well. Many Democrats are eager to reprise their successful midterm election strategy of pointing to Republican efforts to repeal and replace the ACA.

President Trump and his allies, in contrast, are looking to counter with attacks casting Medicare-for-all as a scary takeover of the health-care system that would be detrimental to many Americans, a line of attack that worries some Democrats. The result could be a campaign in which each side tries to terrify voters about the other side’s plans.

“If the battlefield is on the ground of health care, we will start with an advantage,” said Ian Russell, a former deputy executive director of the Democratic Congressional Campaign Committee. “However, depending on who our nominee is, we could have some vulnerabilities.”

With AARP holding candidate forums this week in Iowa, and the next Democratic debate coming up at month’s end, the candidates are facing more pressure to detail their positions on health care. Sen. Cory Booker (N.J.) released a proposal Monday that would expand eligibility for Medicaid. And on Tuesday, Harris is expected to introduce a plan to reduce prescription drug prices.

At the heart of Biden’s health-care plan, which senior campaign officials said would cover more than 97 percent of Americans, is a proposal to let people choose a government-run health system like Medicare if they aren’t happy with private insurance. President Barack Obama initially set out to include such a public option in the ACA legislation, but later backed away from the idea amid political resistance.

The former vice president also would bolster other parts of the ACA designed to help people buy insurance. It would get rid of the income limit — 400 percent of the federal poverty level — used to determine who qualifies for tax credits that help Americans pay insurance premiums.

Biden’s plan also would seek to circumvent the resistance by many Republican-led states to accepting the expansion of Medicaid, a program for low-income and disabled Americans. In the 14 states that have not expanded Medicaid as allowed by the ACA, Biden’s proposal would let people who would otherwise qualify for assistance buy into the public option without premiums.

In an effort to strengthen abortion rights, Biden would provide federal funding for Planned Parenthood and combat the actions of states that have moved to tighten restrictions on the procedure.

His senior campaign officials estimate that the plan would cost $750 billion over 10 years, to be funded by raising taxes on the investment income earned by wealthy Americans, the officials said.

After initially aiming most of his fire at Trump, Biden now appears eager to contrast his position on health care with those of his Democratic rivals. Campaigning in New Hampshire last week, Biden said that it “took a long time to get us to where it is now,” referring to the decades-long battle to pass health-care restructuring. Too many people are at risk, he warned, to go through another long debate about overhauling the system.

He went after proponents of Medicare-for-all both directly and obliquely. Biden said Sanders has “been very honest” about raising taxes on the middle class to pay for his plan and ending private insurance, suggesting that those ideas are political poison.

“I have great respect for my opponents. I think their heart is in the right place — trying to make sure that health care is a right in America, and that health care is decent here,” Biden said Monday in Iowa.

Medicare-for-all is the latest version of what has traditionally been called a single-payer system, in which the government would provide insurance to all Americans, replacing private insurance for basic coverage. Such government-run health care is the norm among almost all industrialized nations.

Sanders acknowledges that his proposal would require a tax increase on middle-class Americans, but says they’d pay far less in health-care costs, saving them significant money overall. And although his plan would bar private insurers from replicating services covered under Medicare-for-all, those firms could still cover elective and supplemental procedures.

The debate about the future of private insurance has been complicated, however. Some Democrats have tried to walk a fine line, endorsing Sanders’s plan while trying to avoid alienating Americans who want the option to keep their insurance. Harris, who is co-sponsoring Sanders’s Medicare-for-all bill in the Senate, recently had to clarify that she does not support abolishing private insurance.

Sanders’s aides say a core part of their strategy is to emphasize that the senator has spent decades advocating for ideas that have only recently become popular with other Democrats. In his speech Wednesday, Sanders intends to “confront the Democratic opponents of Medicare-for-all and directly challenge the insurance and drug industry,” his campaign said.

On Monday in Philadelphia, Sanders defended his ideas. “Under Medicare-for-all, we are going to invest in health-care facilities that operate in underserved areas,” he said.

Later, the Sanders campaign released a document comparing his plan to Biden’s. “A ‘public option’ would be both a policy and moral failure at a time when the American people want Medicare-for-all,” said Faiz Shakir, his campaign manager.

And over the weekend, Sanders was just as eager to focus on Biden as Biden was to name him.

“Here are the facts. Under Medicare-for-all, over a four-year period, we will transition to a system in which Medicare is expanded to cover every man, woman, and child in the country,” Sanders said Saturday in a statement responding to Biden. He added, “It is preposterous to argue that as we expand Medicare-for-all that people with cancer and other illnesses will not get the care that they need.”

There is evident support among Democrats for both Medicare-for-all and the ACA, leading many in the party, including some of the presidential hopefuls, to stake out ground between the two options: voicing support for the former as an aspirational goal, while underscoring the need to protect and build upon the latter.

On the whole, however, Democrats prefer a universal health-care system run by the government to the current one, polling suggests.

More than three-quarters of Democratic-leaning adults — 77 percent — said they favored a universal program, and 66 percent supported this option even if no private insurance were available, according to a recent Washington Post-ABC News poll.

Among the general public, the poll showed that 52 percent backed the government-controlled universal system, although only 43 percent said they would support it if no private insurance were available. Other surveys have shown that responses to Medicare-for-all vary widely based on the information provided.

When it comes to the Affordable Care Act, 46 percent of the public held favorable views of the law, while 40 percent held unfavorable views — a major improvement for the ACA’s standing from several years ago but a slight dip from the spring, according to a recent Kaiser Family Foundation poll.

One thing Democrats agree on is that health care is likely to be a major focal point in the primary and general elections.

“I think health care is going to be a preeminent issue,” said Sen. Richard Blumenthal (Conn.), who like many in his party has associated himself with different options for improving the health-care system. “Wherever I go in Connecticut, people say to me the system right now is broken and something needs to be done.”

John Wagner, Chelsea Janes and Scott Clement in Washington and Cleve R. Wootson Jr. in Des Moines contributed to this report.