Think of it as Super Postponed Day.

June 2 had been an afterthought on the Democratic primary calendar. Ever since Joseph R. Biden Jr. seized the mantle of front-runner, voters in New Jersey and a few other states scheduled to vote that day assumed the Democratic horse race would be over before their primaries rolled around.

But with numerous states pushing back voting to June 2 because of the coronavirus pandemic, the date has gained sudden prominence. It now confers a huge bounty of delegates, second only to Super Tuesday in early March, with Indiana, Pennsylvania and others moving to hold their primaries on the first Tuesday in June.

Although Mr. Biden has built an all but insurmountable lead, June 2 — which is a long 10 weeks away — will be his first chance to clinch the presidential nomination. Only then would the former vice president have a definitive reason to press for the withdrawal of Senator Bernie Sanders of Vermont, who has shown no inclination to leave a race that feels frozen in place.

Some Democratic strategists see possible perils in the delay. Having to wait until June 2 for the next major chapter in the nominating race largely deprives Mr. Biden of a chance to rack up interim victories that would bring media attention; President Trump, meanwhile, is promoting his leadership in a global pandemic. A Monmouth University Poll on Tuesday showed Mr. Biden with a three-point lead over Mr. Trump among registered voters nationally, 48 percent to 45 percent, an edge the pollsters called “negligible.”

As the Democratic nominee-in-waiting, Mr. Biden may simply have to wait longer.

“This idea apparently being floated by the Sanders campaign that Bernie can stay in the race and accumulate delegates without harming Biden’s chances of winning in November is delusional,” said J.J. Balaban, a Democratic strategist in Pennsylvania. The lengthy pause in the primary and Mr. Sanders’s reluctance to step aside are distractions, Mr. Balaban said, from the need for Democrats to refocus on the general election race, in which Mr. Trump has a head start raising money and coalescing supporters.

Michael Soliman, a Democratic strategist in New Jersey, said the delay might not be all bad if it concentrates the minds of voters on the need for change in the White House. “As this crisis causes more states to push back their primaries, New Jersey voters will not only be reminded about the need for strong and steady leadership, but they will finally be in a position to play a meaningful role in securing the nomination for Joe Biden,” he said.

On Tuesday, Delaware became the latest state to move its primary, originally scheduled for April 28, to June 2, under an executive order by Gov. John Carney. Pennsylvania’s Legislature passed a bill on Wednesday to likewise reschedule the state’s primary from April 28 to June 2, which Gov. Tom Wolf is expected to sign by the end of the week.

Delaware and Pennsylvania would bring to six the number of states to move primaries to June 2, including Connecticut, Indiana, Maryland and Rhode Island. The newcomers join the five primaries originally scheduled that day: New Jersey, Montana, New Mexico, South Dakota and the District of Columbia. (Inevitably, the date seems destined to be stamped the “Acela Primary” for its new regional tilt.)

Ohio, which canceled its primary scheduled for March 17 because of the coronavirus, also has its sights on June 2. The date is one week before the June 9 deadline that the Democratic National Committee has set for states to hold primaries.

Election officials in New York, which is emerging as a hot spot of infections, are considering delaying their presidential primary from April 28 to June 23. A group representing New York’s election commissioners on Tuesday called for the primary to be moved to June 23, the date of another statewide vote, and to allow all voters to request absentee ballots. As the D.N.C. rules currently stand, New York could forfeit some delegates at the national convention for voting so late.

In all, 822 pledged delegates would be at stake on June 2 if all the states eyeing that date follow through. The D.N.C. has said a candidate needs 1,991 to clinch the nomination.

Mr. Biden currently leads, with 1,214 pledged delegates to Mr. Sanders’s 910. The delegate math has become nearly prohibitive for Mr. Sanders; to overtake Mr. Biden, he would need to win the remaining states by about 20 percentage points.

Few of the remaining states on the map feature the voting blocs, chiefly large numbers of Hispanics, where Mr. Sanders has done well. He has lost in a series of states where he was thought to have the advantage, notably Michigan, Minnesota and Texas.

With the coronavirus outbreak, the Vermont senator has turned away almost entirely from conventional campaigning, instead converting his campaign apparatus to rally his liberal supporters around fighting for his vision of health care during the emergency. On Tuesday night he streamed a “coronavirus town hall” featuring health experts, a musical guest and Representative Pramila Jayapal of Washington, a chair of the Congressional Progressive Caucus.

Four years ago, Mr. Sanders never formally dropped out of the primary race even after Hillary Clinton became the presumptive nominee on June 7. (Mr. Sanders endorsed Mrs. Clinton a month later, just before the national convention.)

Mr. Biden has struggled to adapt to the new political landscape in which there are no more scheduled debates or live rallies, and Mr. Trump has seized the national spotlight with daily briefings from the White House. A Biden appearance from his Delaware basement on Monday, in which he criticized Mr. Trump for not acting like the “wartime president” he claims to be, was not picked up in real time by any of the major cable news channels.

Mr. Biden appeared again from his new home studio in a round of television news interviews on Tuesday. On MSNBC, he said that he opposed calling off any elections; on CNN, he said that he was “staying in place, not gathering at events” and that anyone who enters his home, Secret Service agents included, wears masks and gloves.

He was asked about Mr. Sanders’s “ramping up” for primaries in New York and elsewhere.

“I think that’s his decision,” Mr. Biden said.