A Democratic debate that was tamed from the outset by coronavirus-related precautions nevertheless produced fireworks Sunday night as Bernie Sanders repeatedly attacked Joe Biden’s lengthy record and spurned the presidential primary front-runner’s attempts to win over him and his base by adopting liberal platforms.

Biden, meanwhile, bristled at Sanders’ transparent attempts to use the coronavirus pandemic to renew his call for “Medicare-for-all,” saying: “People are looking for results, not a revolution.” But the Vermont senator was notably more aggressive, hitting his rival in their first one-on-one debate over everything from super PAC support to his history on Social Security.

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Throughout, Biden staked out unusually left-wing positions for a front-runner -- including by promising that no one would be deported in his first 100 days in office, and by asserting that there would be "no new fracking" under his administration. Fracking, while opposed by environmental activists, has revitalized the economies of battleground states and is expected to be a major issue in the general election.

Even with no live audience, the debate had lively moments. At one point, Sanders urged viewers to "go to the YouTube" to review evidence of Biden calling for cuts to Social Security on the Senate floor. That prompted one of the night's most contentious exchanges, as Sanders pinned down Biden aggressively -- with Biden eventually acknowledging under pressure that various cuts were "on the table" as part of congressional negotiations, but that he "never voted" specifically to slash Social Security.

"We did not cut it!" Biden exclaimed during the exchange, hammering his lectern.

"I know, because people like me helped stop that!" Sanders retorted. "You're an honest guy, why don't you just tell the truth here?"

Biden has, in fact, previously supported temporary freezes on Social Security benefit increases, though has largely opposed cuts.

Their first one-on-one stand-off represented perhaps Sanders’ last moment to alter the trajectory of Biden’s front-running campaign ahead of another round of primaries that could see the Vermont senator fall further behind in the delegate count. Biden, meanwhile, is essentially looking for a peace accord, as the party establishment aims to avoid a drawn-out and bruising primary battle redolent of 2016.

Speaking to these concerns, Biden said at one point he’d back Sanders and campaign for him if he’s nominated, then added: “I would hope that Bernie would do the same thing if I’m the nominee.”

The underdog showed little interest in holding back. In another heated moment, Sanders pressed Biden on accepting money from super PACs: "Why don't you get rid of your super PAC that you have right now which is running very ugly negative ads against me?" he asked.

Biden then laughed, prompting Sanders to respond, "Don't laugh, Joe, that's just the truth."

The former vice president shot back that Sanders should get rid of the "nine" super PACs he has, and offered to list those super PACs.

When Sanders called that bluff and urged Biden to "list" the supposed super PACs, Biden refused and simply told Sanders to "come on, man."

Biden also criticized Sanders for voting against a package that included Russia sanctions in 2017, although Sanders' vote was intended to avoid the package's sanctions on Iran in order to save the Obama administration's nuclear deal with that country.

At various moments in the event, each candidate tripped up over key terms, as Sanders confused Ebola with the coronavirus, and Biden called H1N1 the "N1H1" virus and referred to Ebola only as "what happened in Africa."

The tensions at times left Biden clearly frustrated, at one point saying he had been trying to give Sanders “credit” for some things in his record but he was making it difficult.

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As the debate calmed somewhat, Biden made news by promising that his prospective vice presidential pick would be a woman. Sanders said "in all likelihood" he would do the same, as long as the woman was sufficiently progressive.

Biden also promised that in his first 100 days as president, "no one will be deported at all," a vow that would also presumably protect murderers or rapists who violate federal immigration law.

The debate took place in a Washington studio, and the candidates were kept six feet apart, steps taken in response to the novel coronavirus outbreak. Yet Sanders at times still appeared as if he were addressing a crowd, urging the public to fact-check Biden’s claims on stage.

And President Trump also came under fire for his coronavirus response.

Earlier in the debate, arguing we need to "shut this president up right now" because he was "blabbering with unfactual information which is confusing the general public," Sanders went on to call the moment a referendum on the nation's health care system as he renewed his call for "Medicare-for-all." Biden, however, cast the crisis as a probative test of presidential leadership.

"First of all, the dysfunctionality of the current health care system is obviously apparent," Sanders said. "As I said earlier, there are people who hesitate going to the doctor. ... Clearly we are not prepared, and Trump only exacerbates the crisis. When we spend twice as much per capita as any other nation, one might expect we're gonna have enough doctors all over this country. One might expect we're gonna have affordable prescription drugs."

He added: "What the experts tell us is that one of the reasons we're unprepared is we don't have a system. We have thousands of private health insurance plans. That's not a system that is prepared to provide health care to all people."

"It is not working in Italy right now, and they have a single-payer system," Biden countered. “It has nothing to do with Bernie’s ‘Medicare-for-all.’”

"I don't want to get this into a back and forth in terms of our politics," Biden bizarrely remarked at one point. He also asserted that ad hoc coronavirus response legislation currently under consideration by Congress was the appropriate response to take care of the "immediate needs we have now."

Medicare-for-all, Biden said, had no realistic chance of passing Congress, and likely couldn't be funded.

"We're responding," Biden said. "It's all free, you don't have to pay for a thing. That has nothing to do with whether or not you have an insurance policy. This is a crisis. We're at war with the virus."

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"That's not true, "Sanders interjected. "That law has enormous loopholes. I understand Nancy Pelosi did her best, Republicans prevented it."

Sanders went on to argue that "right now, we have a lack of medical personnel," and that "unprecedented action" was needed. The self-described "democratic socialist" urged the Trump administration to promise Americans that "if you lose your job, you will be made whole."

He then suggested that the government can guarantee workers' wages if it can inject $1.5 trillion into short-term money markets -- a comparison that economists have said is inapt.

Biden, meanwhile, said first responders lacked necessary "equipment" to handle the crisis. He called for widespread access to testing kits and using federal resources to add more hospital beds.

“This is bigger than any one of us,” he said.

Although the debate hall had no live audience, it generated strong reactions immediately in Washington. Rep. Alexandria Ocasio-Cortez, D-N.Y., tweeted during the debate that Biden's positions, including his ostensible support for a fracking ban, were disingenuous. She also hit the former vice president for abruptly changing his opinion on the Hyde Amendment, which generally prohibits federal funding for abortion, and for minimizing his role in passing the 2005 Bankruptcy Bill that limited borrower protections.

The candidates separately tangled again over Sanders' widely panned comments praising former Cuban dictator Fidel Castro's "literacy programs," which alienated even some Democrats, especially in Florida. "Words matter," Biden remarked.

The debate comes as the nation and much of the world struggle to contain a global pandemic amid mounting economic uncertainty. The debate itself was upended by the outbreak, having been moved from Phoenix to a Washington studio.

For Biden, meanwhile, he has to strike a challenging balance – appealing to a general election audience while also making concessions to the left, as the party tries to coax Sanders out of the race.

Hours before the CNN-Univision debate began, Biden announced he would adopt portions of two proposals from Sanders and Elizabeth Warren concerning free public college and bankruptcy, respectively, in an apparent bid to win over their supporters.

But, Sanders' campaign quickly rebuked the Biden initiative, saying it simply wasn't sufficient. The shift was somewhat unusual for Biden, as front-runners typically have moved toward the political middle with an eye toward November, instead of embracing more left-wing ideas. But here, Biden clearly is trying to demonstrate to Sanders as well as his base that he embraces at least part of their progressive agenda, likely in hopes of convincing Sanders and his followers to support him.

Much has changed in the United States, and in the campaign, since the Democrats' last debate less than three weeks ago. Biden stepped on stage as the front-runner, a distinction that seemed unlikely when Sanders was winning early contests. But, more moderate Democrats have consolidated rapidly around Biden, buoyed by his strong standing with black voters and motivated by a desire to block Sanders, as many of them signaled fears he'd lose to Trump in November.

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The fast-moving coronavirus largely has grounded the candidates for days, leading them to cancel campaign rallies even as big states prepare to vote on Tuesday. That's also frozen the trajectory of the primary contest, limiting Sanders' opportunities to regain momentum.

After rebounding in stunning fashion from his sluggish start in the primary, Biden has held a solid lead over Sanders in the all-important delegate race since. A strong showing in Tuesday’s primary contests essentially could guarantee his nomination. Four big states will be up for grabs: Illinois, Ohio, Arizona and Florida, a perennial general election battleground where Biden has appeared to have an edge over Sanders.

The Associated Press contributed to this report.