“We are still largely people who need that sort of human element to it,” said Karen Finney, a longtime Democratic strategist who has advised Hillary Clinton and Stacey Abrams. “There is nothing that replaces in-person events.”

The premier in-person event in democracy, historically — voting itself — has also been scrambled. Two states, Louisiana and Georgia, are postponing their primaries amid virus concerns. Others, including Florida, Illinois, Ohio and Arizona, plan to go forward with their elections this week.

But for the candidates competing in those contests, confinement to a campaign live-stream is a significant impediment.

The past four presidents, for their many differences, all distinguished themselves with an ability to excel in a room, either through soaring speechmaking (Barack Obama), rampaging rally (Mr. Trump) or a talent for insta-connections with voters in smaller, unrehearsed settings (Bill Clinton and George W. Bush).

Certainly all three major 2020 candidates are being stripped of some signal political strengths during the hiatus. Mr. Trump’s events are something akin to oxygen for him, rousing his base and supplying the live-action feedback and adulation he craves.

Mr. Biden’s best moments often come on the voter rope-line after he leaves the lectern, showcasing his empathy and charm — and his eagerness to shake hands, shoulders, anything in range — more effectively than his public remarks tend to. (Other flourishes, like his recent profanity-specked exchange with a man in Michigan, who suggested Mr. Biden wanted to confiscate guns, can also flow from these interactions, for better or worse.)

The Sanders campaign has been perhaps the nimblest and most experienced with virtual gatherings and social media-driven events long before the outbreak, powered in part by the youth and tech savvy of the senator’s core supporters. But Mr. Sanders is now nonetheless robbed of a crucial data point as he lags Mr. Biden in delegates: the zeal and attendance at his rallies, which allies hold up as evidence that his revolution is resonating widely, even if the primary results have not always agreed.