Former President Donald J. Trump’s performance in Tuesday night’s debate is likely to exacerbate the widening financial gap that he faces with Vice President Kamala Harris, who is now enjoying her eighth week of momentum with big and small donors alike.

Several large donors and fund-raising activists for Mr. Trump, along with their advisers, described the debate as a significant missed opportunity for him to firm up his support with voters and major contributors. At the Republican National Convention in Milwaukee in July, Mr. Trump’s financial backers were giddy to the point of almost overconfident. Now, his wealthy supporters are confronting the possibility that they may be dramatically outspent.

Debates are pivotal to both high- and low-dollar fund-raising. In interviews late on Tuesday and on Wednesday, while Mr. Trump’s backers echoed his complaint that the debate moderators had been unfair to him, many also said that he should have persisted regardless and that they believed he had lost the encounter.

Doug Deason, a Texas fund-raiser who watched the debate with his billionaire father, Darwin Deason, described the debate as “painful to watch” given his distaste for Ms. Harris and his frustration with the moderators and with Mr. Trump’s undisciplined performance.

“He just can’t sit and discuss policy,” Mr. Deason said. “He had many opportunities to expose her lies and failed on most.”

Keith Rabois, a venture capitalist who along with his husband has donated millions of dollars to pro-Trump groups, said he had fielded nervous messages from friends about Mr. Trump’s debate performance up until his closing statement, which Mr. Rabois considered strong.