WASHINGTON — The Democratic National Committee on Thursday postponed its presidential convention because of the coronavirus, moving it from mid-July to mid-August, and making it the largest political event to be upended by the public health crisis sweeping the country.

The convention will still be held in Milwaukee, as planned, the week of Aug. 17, officials said — a week before Republicans plan to gather in Charlotte, N.C., to renominate President Trump. The adjacent dates mean that both parties will be using expert health data from the same time frame to assess the safety of bringing together thousands of people at one event — and, ultimately, deciding whether the benefit of staging the conventions outweighs the medical risks.

The timing sets the stage for a high-stakes test of wills and judgment over the next four months between Mr. Trump and former Vice President Joseph R. Biden Jr., the likely Democratic nominee, as the public health crisis paralyzes the country through the summer. Neither will want to be the first to cancel his convention.

The decision to postpone the Democratic convention was made over the previous two days, officials involved in the planning said, but the issue was amplified Wednesday night by Mr. Biden, who called for rescheduling the convention during an appearance on “The Tonight Show.’’

Tom Perez, the chairman of the Democratic National Committee, said he did not consider moving the event to September, after the Republicans, and was averse to conducting a virtual convention that would be less likely to attract hours of television coverage.

“We’re going to hold a very exciting and safe convention in Wisconsin to highlight our nominee and to make sure that people know the values of the Democratic Party and what we’re fighting for,” he said in a telephone interview.

Advisers to Mr. Biden and senior party leaders stressed Thursday that they remained flexible on the format and that they would be guided by the principle of safety first.

But campaign officials and other Democrats said Mr. Biden’s team also wanted to preserve the prospect of a large, in-person convention whose pageantry and media coverage could provide a boost heading into the final months of the general election.

Mr. Trump has made clear his desire to proceed with plans for a celebratory convention that dominates the airwaves, and Republicans are determined to provide it. That resolve has in turn has put more pressure on Mr. Biden, who was already in need of ways to draw attention from a president with a fearsome bully pulpit. Democrats know that canceling one of their most prominent events could yield a significant advantage Mr. Trump just two months before the election.

Presidential challengers always prize three moments of dayslong publicity — with their vice-presidential selection, their conventions and in the fall debates. That sustained coverage is all the more important now that the virus is likely to keep Mr. Biden off the campaign trail for much of the spring and may substantially condense the general election period.

“There are things you can do online, but there is something about the communal spirit that is important and people feed off of,” said David Axelrod, a senior official in Barack Obama’s presidential campaigns. “From a television standpoint it’s hard to have the same impact if you’re cobbling together a series of remotes.”

Though an in-person convention would provide Mr. Biden a nationally televised boost, the later date carries potential downsides. In addition to potentially keeping Mr. Biden out of the spotlight for an extended period of time, it could complicate his timing on when to announce his vice-presidential choice. And it delays for a month his ability to access funds earmarked for the general election.

Concerns over the virus have already led to the cancellation of hundreds of state and local conventions from both parties. Moving the national convention back a month is an acknowledgment that the outlook for holding an in-person gathering in July — with some 4,500 delegates, and tens of thousands of others who convene on the convention city — was not feasible.

“We welcome the D.N.C.’s decision today to prioritize the health and safety of delegates and the greater Milwaukee community by postponing the Democratic National Convention until August,’’ said Bill Russo, a campaign spokesman, adding that the campaign would continue to work closely with state parties and the D.N.C. “on any changes to the delegate selection process and the format of the convention.”

But there is no guarantee that the crisis will be over by August, and travel could still be difficult or even dangerous for some. One of Mr. Biden’s strongest constituencies is older Americans, the population most vulnerable to the virus, and the lingering effects of the outbreak could still have an effect on which delegates are able to travel.

Dr. Irwin Redlener, a clinical professor at Columbia University’s Mailman School of Public Health, said he was deeply skeptical of a summer convention.

“It is unreasonably optimistic to think that a traditional presidential political convention can happen in the summer of 2020, there’s so much we don’t understand about this,” said Dr. Redlener, who is also the director of the National Center for Disaster Preparedness. He said that gathering large numbers of people together “is counter to every reasonable public health guideline during the pandemic.”

Dr. Redlener was named to Mr. Biden’s public health advisory committee last month, but said he had since stepped aside because of a professional conflict.

Even though other major events scheduled for July had been canceled or postponed, planners had hoped to delay a decision on whether to move the Democratic convention for several more weeks. Mr. Perez told donors as recently as this week that the party had no plans to change the dates, according to people familiar with the conversations.

But Mr. Biden has been receiving extensive briefings from other members of his public health advisory committee. And his remarks on back-to-back nights expressing concern about a July gathering added force to the movement to push it into August.

On Tuesday he said during an interview with MSNBC that it was “hard to envision” the convention taking place as scheduled. The next day he was more forceful, telling the host of “The Tonight Show,” Jimmy Fallon, “I think it’s going to have to move into August.”

“That was all he needed to do,” said Leah Daughtry, who was the chief executive of the 2008 and 2016 Democratic conventions. “When the person who’s the front-runner, and who most people consider to be the person who’s going to get the nomination, expresses that strong an opinion, I think everybody has to pay attention.”

An August convention is likely to be smaller than the planned July event. One senior Democratic official said the event would probably be a “bare minimum” convention, with scores of people who had planned to come staying away either because of health concerns or because they had other plans for mid-August.

Changing the date of the convention will be a daunting logistical feat, requiring the rebooking of thousands of hotel rooms, among other difficulties.

The venue in Milwaukee, the Fiserv Forum, and a nearby convention center are both available the week of Aug. 17, but they are booked the week before with the annual convention of the insurer Northwestern Mutual.

“This is the right decision for the safety of those involved in the convention and for Milwaukee,” said Alex Lasry, a senior official with the Milwaukee Bucks who led the city’s convention bid. “An August convention will provide a much-needed economic boost for Milwaukee and Wisconsin as we come out of this unprecedented time.”

While Mr. Biden has a nearly insurmountable delegate lead over Senator Bernie Sanders, his last remaining rival for the Democratic presidential nomination, he cannot take formal control over convention planning until he clinches the nomination or Mr. Sanders drops out of the race.

Still, he and his team are in the early stages of the search for a potential running mate. The delayed convention could offer more time for the background check process. But some Democrats advised that there were good reasons to announce a vice-presidential pick before the convention.

“I probably would do sooner,” Ms. Daughtry said. She pointed to June, when a host of states are slated to hold rescheduled nominating contests, as a promising moment to announce a choice.

“Folks who want to get energized again, want to turn attention elsewhere, would be energized by a dynamic ticket,” she said. “That’s one reason to do it before these next round of states vote.”

Democratic officials, including Mr. Perez, had hoped and predicted that the party would have a nominee by late April, but with so many states postponing their primaries because of public health concerns about the coronavirus, Mr. Biden cannot clinch the nomination until June at the earliest if Mr. Sanders remains in the race.

In a brief virtual news conference on Thursday, Mr. Biden was asked whether he believed the Wisconsin primary should go forward in person next week, given news that the convention would be delayed.

“I listen to the scientists,” Mr. Biden said via video. “A convention, having tens of thousands of people in one arena, is very different than having people walk into a polling booth with accurate spacing, to — six to 10 feet apart, one at a time, going in and having the machines scrubbed down.”