She added, “I have friends I have to be accountable and explain things to because I told them it was a margin-of-error race. I was promised, Jen O’Malley Dillon promised all of us that Harris would win. She even put videos out saying Harris would win. I believed her, my donors believed her, and so they wrote massive checks.”

Mr. Trump’s resounding victory over Ms. Harris has had Democrats searching for what went wrong.