What happened on Sunday night was not a presidential debate. It was former vice president Joe Biden’s debut as the Democrats’ presumptive 2020 nominee.

The headline of the evening was Biden’s declaration that he plans to pick a female running mate, and that he would nominate an African American woman to the Supreme Court.

But it was the confidence with which Biden conducted himself throughout his two-hour exchange with Sen. Bernie Sanders (I-Vt.) that signaled how dramatically the race has changed in the two weeks since the voters of South Carolina rescued his candidacy from oblivion.

The fact that the debate took place in the shadow of a national crisis only served to illuminate the degree to which Democrats are stepping back from arguing about philosophical differences within their party. Their energy from here on out will be focused on making the case against President Trump, whose handling of the coronavirus epidemic has thrown his prospects for reelection into greater peril.

The stagecraft of the debate underscored how seriously the threat of a deadly epidemic has spun out of control on Trump’s watch. Instead of taking place in Phoenix, as originally planned, it was moved to CNN’s studios in Washington, and the customary audience of cheering supporters was dispensed with (which, it turns out, might be something to consider going forward).

Biden’s and Sanders’s lecterns were set six feet apart, in a nod to the social distancing that health experts recommend, and the candidates greeted each other by bumping elbows, rather than shaking hands.

Still, there were plenty of missed opportunities to demonstrate how the race has shifted into a new, more serious phase. Biden and Sanders spent too much of the evening arguing over long-ago votes.

And Sanders in particular seemed unable to get beyond the familiar refrains of his stump speech. Asked whether he would follow Biden’s lead and commit to putting a woman on his ticket, Sanders stopped short, saying only that “my very strong tendency is to move in that direction.”

Biden, for his part, refused to be drawn into arguing on Sanders’s terms. He shrugged off Sanders’s efforts to make the case that the virus could be better managed if the country had a single-payer health-care system. What the country needs now, he said, is not a revolution but a plan.

“With all due respect to Medicare-for-all, you have a single-payer system in Italy. It doesn’t work there,” Biden said. “It has nothing to do with Medicare-for-all — that would not solve the problem at all. We can take care of that right now by making sure that no one has to pay for treatment period because of the crisis.”

Again and again, Biden came back to his experience in dealing with previous crises as Barack Obama’s vice president. He talked in concrete terms of the steps that he would take were he the president in charge of the response to this one: taking advantage of the test kits developed by the World Health Organization, expanding hospital-bed capacity, providing economic aid to people who may lose their paychecks and small businesses in need of loans to tide them over.

Since his overwhelming victory in South Carolina, Biden — whose campaign was on life support after humiliating defeats in Iowa and New Hampshire — has seen a remarkable consolidation of the Democratic electorate behind him. A newly released Wall Street Journal/NBC News poll found that Biden now enjoys nearly 2-to-1 support over Sanders among Democrats who have voted or plan to vote.

Biden has made a few concessions to the left, a sign that he knows he still has some work to do.

On Sunday, he nodded to Sanders by announcing that he favored making public colleges and universities tuition-free for students whose families earn $125,000 and less. This came after he embraced the proposal of former rival Sen. Elizabeth Warren (D-Mass.) to undo a 2005 law — which Biden as a Delaware senator supported — that made it harder for consumers to file for bankruptcy.

His primary focus, however, has now turned to Trump and the general election. It remains to be seen whether Biden can maintain the steadiness that he displayed on Sunday night. But at a moment when Americans are looking for mature and credible leadership, Biden took a big step toward showing the kind of president he might be.

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