Anyone who thought that former vice president Joe Biden and Sen. Bernie Sanders (I-Vt.) were ready to begin to make peace in their competition for the Democratic presidential nomination got a big surprise on Sunday night. Their debate quickly shifted from talk of pandemics to arguments over past records and future visions.

For much of the two-hour debate, it seemed like business as usual, and yet both candidates and their campaigns know that everything is changing because of the spreading coronavirus. The backdrop to the 2020 election now is one of disruptions to the daily lives of tens of millions of Americans, economic shocks that continue to rattle financial markets and frighten investors and questions about the leadership offered by President Trump.

The setting alone spoke to the extraordinary changes that the coronavirus is forcing on everyone. The debate was scheduled to be held in Phoenix before a live audience. Instead, Sanders and Biden met each other at the CNN studios in the District, with three moderators but no audience. It was a throwback to the first presidential debate of the modern era, when John F. Kennedy and Richard M. Nixon met in the fall of 1960 at a CBS studio in Chicago with no audience.

The ground has shifted dramatically since the last Democratic debate on Feb. 25 in South Carolina. The pandemic has roared into public consciousness, dominating news coverage and altering lives. But the ground also has shifted away from Sanders, and this debate was possibly his last opportunity to slow the momentum Biden has built over the past three weeks.

That created a dual dynamic with the two united in their criticism of the president’s leadership and opposing one another on almost everything else. Sanders in particular used every minute available to draw a contrast with the former vice president. He challenged Biden on a host of issues, cast himself as more visionary than Biden and more consistent in his progressive views.

Early in the debate, he offered a turn-the-other cheek posture. “This is a national crisis,” he said. “I don’t want to get this into a back and forth in terms of our politics here.”

Later, the television audience saw a more indignant Biden, sharply responding to Sanders’s attacks and launching his own.

The debate began on predictable terms, with the two candidates focused on the issue of the moment, the spreading coronavirus pandemic and what they would be doing if they were in the White House rather than Trump. Their policy prescriptions hardly differed and they were generally respectful toward one another. But the discussion of the threats posed by the virus offered a look at the strategies the two candidates were pursuing at the start of the evening.

Sanders used the crisis as a pretext to push the agenda that is at the heart of his candidacy — a sweeping Medicare-for-all plan to fundamentally change the health-care system and attacks on major corporations and their executives. The pandemic only underscored the weakness of America’s health-care system, he argued, and should lead to the kinds of changes for which he has long advocated. Economic assistance should be aimed more at workers than corporations.

Biden was not ready to be drawn into that argument at that point. He focused on the moment, speaking of what he would do now, assuring people that no one should worry about the cost of being treated for covid-19, the disease caused by the coronavirus. “You don’t have to pay for a thing,” he said. “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.”

But once past the issue that now grips the entire nation, it was a rerun of previous encounters over many months of campaigning. They argued, at times testily, over Social Security, the bank bailout during the 2008 financial crisis, campaign money, a bankruptcy bill, abortion, a Green New Deal, gun-control legislation, immigration and, as always, whether Medicare-for-all or adding a public option to the Affordable Care Act would be better for the country.

It is still shocking how quickly fortunes have changed for Biden and Sanders. The last time the Democrats held a debate, there were seven candidates on the stage and Sanders was widely seen as the front-runner for the nomination. Biden was still seeking his first victory of the year after a fourth-place finish in Iowa, a fifth-place finish in New Hampshire and a second-place finish in Nevada.

Then, with a runaway Biden victory in the South Carolina primary four days later, the race was turned upside down. Biden followed up three days later with wins on Super Tuesday, which turned him from a candidate on the brink to the party’s front-runner. Last week’s primaries moved the outcome of the nomination decisively in his direction, leaving Sanders with the hope that Sunday’s debate might somehow reverse fortunes.

Biden is still well short of the 1,991 delegates needed for a first-ballot victory, but with last week’s victories, Biden began to build a lead in delegates that his rival will find nearly impossible to overcome. In two days, he expects to add to that advantage, with primaries in Arizona, Illinois, Florida and Ohio. Four years ago, Sanders lost all four to Hillary Clinton. Given the consolidation around Biden’s candidacy, it would hardly be surprising to see margins of those sizes again.

Then what? The calendar called for a primary in Georgia on March 24, but the state has pushed the date back to May 19 because of the coronavirus. Louisiana was supposed to vote on April 4 but has delayed its contest until June 20. Wyoming, also scheduled for April 4, will hold no in-person voting. Other states are making modifications or considering delays.

The extension of the primary season well into June raises a critical question: Will the postponements in the calendar prompt Sanders to continue his candidacy further into the spring or cause him to decide to shorten his campaigning in the interest of uniting the party and helping the Democrats prepare for the general election?

No group is more eager to know the answer to that than Biden’s team, which last week began to shift its focus increasingly to general-election planning. In normal times, that would mean starting the spade work of staffing up in battleground states, beginning an intensive round of fundraising, assigning priorities and developing as many paths as possible to the 270 electoral votes needed for the nomination.

That’s an enormous task in any presidential election, particularly for a challenger facing an incumbent who has a months-long head start. But almost every aspect of this planning will have to be done with no prior experience for the conditions that now exist.

None of that was addressed during Sunday’s debate, but it is the context for where campaign 2020 is now headed.