Democratic officials have instructed senators, members of Congress and party delegates not to physically attend their national convention this summer, a sign of the ever-shrinking aspirations for their big campaign event in the face of a surge in coronavirus cases in the United States.

The directive, issued Thursday, ensures that little will happen at the convention site in Milwaukee beyond speeches from former Vice President Joseph R. Biden Jr., his vice-presidential nominee and a handful of other top party leaders. The remainder of the events — state delegation meetings, parties, voting on the party platform and on Mr. Biden’s nomination — will happen virtually or not at all.

“We have been working closely with state and local public health officials, as well epidemiologists, and have come to the hard decision that members of Congress should not plan to travel to Milwaukee,” Chasseny Lewis, a senior adviser to the convention committee, wrote in an email to congressional aides. “No delegates will travel to Milwaukee, and Caucus and Council meetings will take place virtually.”

The announcement came on the evening when Mr. Biden, the presumptive Democratic nominee, would have been delivering his acceptance speech for the nomination, underscoring how profoundly the coronavirus has upended the campaign. Earlier this year, shortly after the spread of the virus shut down many states across the country, the Democratic National Committee postponed its event by a month.

“Ensuring the safety and well-being of everyone involved with the 2020 Democratic National Convention drives every decision we make,” said Katie Peters, communications director for the convention committee. “This communication reiterates our guidance from several weeks ago that all members of state delegations — including elected leaders — should plan to conduct their official business remotely.”

Traditionally, conventions have marked the start of the heated fall campaign season, giving parties a chance to energize their supporters and sway undecided voters with days of pageantry and largely uninterrupted media coverage. Party leaders typically spend months — if not years — fund-raising and planning every aspect of the stage-managed events, from the program to the swag to the balloon drop that traditionally ends the celebrations.

Over the past several months, Democrats have scaled back their plans, moving the convention across downtown Milwaukee from Fiserv Forum, the city’s professional basketball arena, to the Wisconsin Center, a modest convention space that typically hosts events like the city’s car show. The last national political event to take place there was the 2004 Green Party convention.

Party officials have been consulting regularly with doctors and epidemiologists and say they are following the recommendations of the medical establishment.

Their decision comes as Mr. Trump has made clear he wants the Republican convention to go on,. Party officials moved it to Jacksonville, Fla., from Charlotte because North Carolina state officials said they would have to abide by social distancing rules.

On Thursday, the Republican National Committee announced new details about the convention, confirming a Times report this week that the party was planning to shift some events to take place outdoors.

Ronna McDaniel, the committee’s chairwoman, said that “a number of indoor and outdoor venues” in Jacksonville would be used, including the VyStar Veterans Memorial Arena, where the indoor program had been scheduled to take place, as well as several other locations. For the first three days of the convention, attendance will be limited to regular delegates, while a broader group will be allowed on the final day when Mr. Trump accepts the party’s nomination.

Many top Republicans are skipping the event, flouting Mr. Trump’s desire for an elaborate event attended by large crowds.