Few people outside Ms. Pelosi’s inner circle were aware of how worried she was that Mr. Trump would try to stop the opposition party from taking control of the House unless the Democrats’ victory was emphatic enough to be indisputable.

“If we win by four seats, by a thousand votes each, he’s not going to respect the election,” said Ms. Pelosi, recalling her thinking in the run-up to the 2018 elections.

“He would poison the public mind. He would challenge each of the races; he would say you can’t seat these people,” she added. “We had to win. Imagine if we hadn’t won — oh, don’t even imagine. So, as we go forward, we have to have the same approach.”

In recent weeks Ms. Pelosi has told associates that she does not automatically trust the president to respect the results of any election short of an overwhelming defeat. That view, fed by Mr. Trump’s repeated and unsubstantiated claims of Democratic voter fraud, is one of the reasons she says it is imperative not to play into the president’s hands, especially on impeachment.