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.”