WASHINGTON — One of the thorniest debates in American politics is over health care. Now add a pandemic.

The future of America’s health insurance system has already been a huge part of the 2020 presidential campaign. At campaign events over the past year, voters have shared stories of cancer diagnoses, costly medications and crushing medical debt.

That was before more than 63,000 people in the United States tested positive for the coronavirus, grinding the country to a halt, upending lives from coast to coast, and postponing primary elections in many states. The virus has made the stakes, and the differing visions the two parties have for health care in America, that much clearer.

“Health care was always going to be a big issue in the general election, and the coronavirus epidemic will put health care even more top of mind for voters,” said Larry Levitt, the executive vice president for health policy at the Kaiser Family Foundation, a nonpartisan research organization. “Sometimes these health care debates can get a bit abstract, but when it’s an immediate threat to the health of you and your family, it becomes a lot more real.”

On Monday, Joseph R. Biden Jr. sent a letter to President Trump and Republican state officials that emphasized the sorts of immediate threats Americans are feeling, and criticized those Republicans for supporting litigation that targets the Affordable Care Act. The letter called it “unconscionable that you are continuing to pursue a lawsuit designed to strip millions” of coverage in the midst of a pandemic.

Mr. Biden sent his missive on a health care milestone: the 10th anniversary of when President Barack Obama signed the Affordable Care Act into law — with Mr. Biden, then the vice president and now the likely Democratic presidential nominee, standing by his side.

While the Democrats spent much of their primary fighting about whether to push for “Medicare for all” or build on the Affordable Care Act, the coronavirus crisis may streamline the debate to their advantage: At a time when the issue of health care is as pressing as ever, they can present themselves as the party that wants people to have sufficient coverage while arguing that the Republicans do not.

“A crisis like the coronavirus epidemic highlights the stake that everyone has in the care of the sick,” said Paul Starr, a professor of sociology and public affairs at Princeton who served as a health policy adviser in the Clinton White House. “It really strengthens the Democratic case for expanded health coverage, and that should work, I should think, to Biden’s advantage in a campaign against Trump.”

The virus is also having dire economic consequences, depriving Mr. Trump of a potent re-election argument rooted in stock market gains and low unemployment numbers. It is testing Mr. Trump’s leadership in the face of a national emergency like nothing he has encountered, and if voters give him poor marks, that could inflict lasting damage on his chances in November’s general election.

“It’s a leadership argument,” said Representative Donna E. Shalala, Democrat of Florida, who served as secretary of health and human services for President Bill Clinton. “Who do you want to be president of the United States when there’s a big health crisis?”

In addition to those questions, the virus is a providing an unmistakable reminder that Mr. Trump and Mr. Biden have starkly different views about the future of American health care — and starkly different records on the issue.

Four years ago, Mr. Trump ran for president promising to repeal the Affordable Care Act, popularly known as Obamacare. But his campaign pledge quickly turned into a debacle in the first year of his presidency when Republicans struggled and ultimately failed to repeal and replace the health law. In the midterm elections the next year, Democrats emphasized health care, highlighting issues like preserving protections for people with pre-existing conditions, and they won control of the House.

That line of argument has already surfaced in the 2020 election cycle. “Too many Montana families go to sleep at night worried about health care — coverage, costs, now the fear of coronavirus,” the narrator said in a recent ad targeting Senator Steve Daines, Republican of Montana, who is up for re-election. The ad was run by Protect Our Care, a liberal advocacy group that supports the Affordable Care Act and has set up a coronavirus “war room” aimed at holding Mr. Trump accountable over his handling of the crisis.

Mr. Trump is particularly vulnerable on the issue of health care. Over the course of his presidency, his administration has repeatedly taken steps to undermine the Affordable Care Act, including by arguing in court that the entire law should be invalidated. The Supreme Court agreed this month to hear an appeal in that case, which is the latest major challenge to the law. The court is not expected to rule until next year, but Democrats point to the Trump administration’s legal position as yet another example of the president’s desire to shred the Affordable Care Act.

All together, those steps by Mr. Trump and his administration amounted to something of a policy piñata for Mr. Biden and other Democrats to swing at in the general election, even before the coronavirus threat emerged.

“Trump wants to take health care away,” said Representative Ami Bera of California, a physician. “Democrats and Vice President Biden want to extend health care and make it affordable.”

In his campaign, Mr. Biden has already put a focus on health care, promising to build on the Affordable Care Act and create a so-called public option, an optional government plan that consumers could purchase. On the campaign trail, he has talked about his own exposure to the health care system, including when his late son, Beau Biden, had brain cancer. He has also regularly heard from people about their own struggles. “They walk up and grab me and say, ‘I just lost my daughter, cancer,’ or, ‘My son’s dying,’ or, ‘I have Stage 4,’” he recalled this year.

Andrew Bates, a spokesman for the Biden campaign, said Mr. Trump had “spent almost his entire presidency attempting to cost millions of Americans their health coverage,” adding: “The coronavirus outbreak, which Trump has egregiously mishandled, would be even more catastrophic if he had his way on health care.”

At a Fox News town hall event this month, Mr. Trump said he had not “been able to sell what a great job we’ve done” on health care. While the president and congressional Republicans failed at repealing the Affordable Care Act, they succeeded at undoing a key part of the law when, as part of their 2017 tax overhaul, they eliminated the tax penalty for people who go without insurance.

The Trump campaign has already attacked Mr. Biden over health care, including by arguing that he poses a threat to private health insurance with his proposal to create an optional government plan.

“As President Trump is leading our country and taking unprecedented action to stop the coronavirus, Joe Biden is campaigning on his Bernie Sanders-inspired, socialist health care agenda, which would take away Americans’ access to quality health care,” said Sarah Matthews, a Trump campaign spokeswoman. “Make no mistake about it, Biden’s government-run ‘public option’ is just another name for a government takeover of the entire health care system.”

And although the Affordable Care Act has gained in popularity during Mr. Trump’s presidency, Republicans can still point to rising health care costs as a problem that voters want to see addressed. In that vein, Senator Bill Cassidy, Republican of Louisiana and a physician, cited the substantial premiums and high deductibles that many consumers have.

“Democrats have to have a credible plan to control health care costs,” Mr. Cassidy said. “If you look at what the No. 1 concern is, it is the cost of health care.”

In the Democratic primary race, the health care debate has largely focused on the divide between moderate-leaning Democrats looking to build on the Affordable Care Act and progressives calling for Medicare for all, a government-run health insurance program. Mr. Biden and Mr. Sanders represent the two sides of that argument.

Mr. Sanders, the Vermont senator, faces long odds at catching up to Mr. Biden in the delegate race, but he has remained in the primary and continues to push progressive policy ideas, including on health care. In response to the virus, he has pointed once again to the need for Medicare for all.

“It is nearly impossible to believe that anyone can still think it’s acceptable to continue with a health care system that leaves tens of millions of people uninsured,” Mr. Sanders said this month. “The cruelty and absurdity of that view is more obvious in the midst of this crisis than it has ever been.”

In a poll this month by Morning Consult, four in 10 Americans said the coronavirus outbreak made them more likely to support universal health care proposals in which everyone would receive their health insurance from the government.

Under the single-payer system that Mr. Sanders is proposing, private health insurance would be eliminated — a potential political vulnerability that Republicans would most likely exploit in a general election if the Democratic nominee supported Medicare for all. Mr. Biden, who has repeatedly criticized Mr. Sanders’s health care proposal during the primary, does not share that vulnerability.

At one of his final campaign stops before the virus shut down in-person campaigning, Mr. Biden visited a community health center in Grand Rapids, Mich., where he spoke of his pride in having worked with Mr. Obama to pass the Affordable Care Act.

“I’m running to protect the progress we fought for,” Mr. Biden said. He cast doubt on Mr. Sanders’s plans for a single-payer system and spoke about the urgent need to improve health care — now more evident than even two weeks ago.

Talking about the patients at that clinic, he said, “They can’t afford to wait for a revolution.”