It may also be a last gasp of sorts for the leftist Sanders, who will face mounting pressure to quit if he does not score a major upset this week. Meanwhile, though, the coronavirus pandemic has cast the White House into uncharted territory, leaving the fate of the vote in four states - three of which are among the most populous in the nation and have large delegate numbers at stake - up in the air.
With most aspects of American life upended, the pandemic has achieved the near-impossible by relegating the Biden-Sanders duel to second-tier news. Americans are staying home from work or school by the millions as the country implements emergency measures against a swelling crisis that has already left nearly 70 people dead in the United States and more than 3,500 infected.
Arizona, Florida, Illinois and Ohio have declared they will forge ahead with their primaries despite new health agency guidelines late Sunday that recommend halting any gatherings of 50 people or more. Georgia, which was next in line to vote on March 24, is delaying its primary until May 19. Louisiana had earlier postponed its planned April 4 election to June 20. Both candidates say they will campaign exclusively online for now, a scenario unheard of at the height of a US presidential primary battle.
"We're not going into crowds," Biden said during a one-on-one debate against Sanders, held Sunday night in an empty studio with no live audience. Both men said they are taking full precautions during the outbreak: they are washing hands frequently, their campaign staff is working from home, and live campaign events have been scrapped.
Biden holds a "tele-town hall" at 8:00 pm Monday (0000 GMT) with voters in all four states, when he "will lay out his vision for America and listen to voters' concerns and ideas around restoring the soul of the nation, rebuilding the middle class, and unifying the country," his campaign said. Sanders is scheduled to livestream a "digital rally" one hour earlier for voters in the four states featuring multiple high-profile supporters including the veteran rocker Neil Young and actress Daryl Hannah, Young's wife.
The inability to campaign in person imposes a particular handicap on Sanders, who has consistently mobilized large and enthusiastic crowds at his campaign events. In Sunday's debate, Sanders acknowledged he is now the underdog in the delegate race, but insisted he is "winning the ideological struggle" by drawing voters in the party towards his liberal policy ideas.
The pandemic has exposed "the incredible weakness and dysfunctionality" of America's health care system, he said, seizing on the moment to push his universal health care plan known as Medicare for All. Biden shot back that the plan is unrealistic given today's political divisions, and that Sanders has yet to explain how he would pay for such a costly reshaping of the economy.
"He still hasn't told you how he's going to ever get it passed. He hasn't told you how in fact there's any possibility of that happening," Biden added. "People are looking for results, not a revolution," Biden said, referring to Sanders' call for radical socio-economic change.