“If we’re fortunate enough to host the convention and playoffs, we’ll figure out how to make it work,” said Alex Lasry, a senior Bucks official who led Milwaukee’s convention bid. “These will be two of our biggest events, so we’re going to figure it out, make them successful and put Milwaukee on the map.”

Rewriting a convention plan on the fly is far easier for a party with a presumptive nominee and no possibility of challenges on the floor, which can be pesky issues under normal circumstances. Mr. Trump’s allies on the Republican National Committee have seen to it over his three years in office that anyone who might be a dissenting voice in Charlotte — where the N.B.A. team that plays in the convention arena is far out of playoff contention — is kept away from important roles in the party or its convention.

“The 2020 Republican convention is going to be the coronation of Trump,” said Scott W. Reed, the senior political strategist for the U.S. Chamber of Commerce, who also directed preparations for the Republican convention in 1996.

Mr. Reed said he saw little downside to hosting a virtual convention, if it comes to that.

“Trump’s four most favorite political words are: never been done before,” he said. “It doesn’t mean you wouldn’t give a major address, but you could really focus it on the six battleground states and highlight events in those states throughout the evening.”

While Republicans have little to debate at their convention, Democrats are bracing for fights. Allies of Senator Bernie Sanders, who remains in the race despite Mr. Biden’s commanding advantage, are encouraging him to keep going to accumulate more delegates. That would enable him to influence the platform and rules discussions, debates that animated the 2016 Democratic convention.

Larry Cohen, a longtime Sanders ally who is chairman of Our Revolution, the political organization that sprung from the 2016 Sanders campaign, published an op-ed Monday in the liberal magazine In These Times warning that if Mr. Sanders drops out, his allies may not win any seats on the powerful rules and platform committees at the convention. That prospect, he wrote, could lead to the party resurrecting the power of superdelegates for the 2024 primary election, an arrangement Mr. Sanders denounced in 2016 and fought vigorously to change.

And while Republican convention delegates will be a who’s who of Trump supporters without meaningful opposition, a virtual Democratic convention would leave would-be Sanders delegates without much prospect of pushing the party to the left.