According to an internal memo made public as part of a court case, the Trump campaign knew as early as November that its outlandish fraud claims were false. Last week, the panel showed videotaped testimony of his top advisers and even the attorney general at the time, William P. Barr, saying that they had told Mr. Trump and top White House officials as much.

Mr. Stepien was present for key conversations about what the data showed about Mr. Trump’s chances of succeeding in an effort to win swing states, beginning on election night. He was part of a meeting with Mr. Trump on Nov. 7, 2020, just after the election had been called by television networks in favor of President Joseph R. Biden Jr., in which he told Mr. Trump of the exceedingly low odds of success with his challenges.

Mr. Trump, urged on by his lawyer, Rudolph W. Giuliani, wanted to press forward anyway.

Mr. Stepien, who rarely speaks in public, is appearing under subpoena, raising questions about how willing a witness he will be against Mr. Trump.