In 2020, Democrats opted to hold their convention virtually because of the coronavirus pandemic, canceling on the city of Milwaukee as hotels and businesses emptied across the country. Republicans staged part of theirs at the White House, a radical break from tradition that mixed politics with the trappings of governance and prompted Democrats to charge the Trump administration with a litany of Hatch Act violations. (The law generally bars government employees from participating in partisan activities.)

Eight years later, Mr. Trump is a known quantity, and his presence is far more of a complicating factor. One Milwaukee bar, the Mothership, announced last month that it was closing down for the convention, because, the owner, Ricky Ramirez, said, “I’m not trying to get involved with or actively take money or rent the space out to that tomfoolery.”

Some organizations will have a toehold in Milwaukee as they go all in on Chicago. The Latino Leaders Network will hold a reception at a Milwaukee law office for about 150 people, said the group’s chairman, Mickey Ibarra. It is holding a blowout on Navy Pier for 750 guests, with a contract that allows it to expand to 1,000.

“There will be a very big difference,” he said.

But even in Chicago, there is some concern that, beyond labor unions, state delegations and liberal interest groups, major corporations could be shying away from the Democratic convention, because those businesses are also shying away from the Republicans, said Sam Toia, the president of the Illinois Restaurant Association. As in Milwaukee, conversations are happening with such companies, but contracts have yet to be signed.

Skittishness from deep-pocketed corporations may be keeping some of the largest, priciest venues out of the conversation. The Milwaukee Art Museum, with its panoramic views of Lake Michigan designed by the renowned Spanish architect Santiago Calatrava, will not be hosting private parties, for instance, convention and Milwaukee officials said, citing cost and a desire not to entangle the museum with politics.