‘He Stiffed Our Party’: Bloomberg Doubts Resurface Before D.N.C. Speech

“There’s no sane, rational person in politics who would say, ‘I’m going to spend “X” amount of money on a race,’” Mr. Nutter said, adding that much of Mr. Bloomberg’s influential giving around the 2018 midterm elections had come the month before Election Day. “He’s usually a later supporter, when we can really figure out the landscape.”

Yet while “donors do like to hold their cards till the 11th hour,” said Dr. Powell, the political science professor, large gifts that arrive early can allow campaigns to make better strategic choices, especially this cycle. “Our normal election timeline really has to be shifted forward, given there’s going to be so much early voting,” she said.

Some Bloomberg allies suggested that he had simply decided that Mr. Biden didn’t need his assistance, given the former vice president’s lead in the polls and the increasing strength of his fund-raising operation, which has at times outpaced Mr. Trump’s.

Indeed, asked in January about his potential future support of the Democratic nominee — as he indicated a willingness to open his wallet to whomever the party chose — Mr. Bloomberg implied that he would calibrate his giving. “It depends whether the candidate needs help,” he said. “If they’re doing very well, they need less. If they’re not, they’ll need more.”

Mr. Bloomberg’s transfer to the party in the spring went toward building out field operations in battleground states like Florida, Pennsylvania and Virginia earlier and faster than anticipated, though a party spokesman declined to specify how that money had been spent.

“He’s continuing the work he’s done over several cycles to support our party, and coming out of the convention, he’s helping put Democrats in the strongest position possible to win at every level of the ballot in November,” said Tom Perez, the chairman of the Democratic National Committee.

Stephanie Cutter, the convention’s program executive, said Mr. Biden wanted Mr. Bloomberg to speak specifically about the state of the economy. “Mayor Bloomberg worked with Joe Biden during the economic recovery 10 years ago and saw firsthand Biden’s ability to move the country out of this crisis,” Ms. Cutter said.