William Dreher, a prosecutor, got him to admit several times that Mr. Trump had not been at his side, offering him step-by-step instructions, when he walked into the Senate parliamentarian’s office and walked out with the whiskey and the coat rack. Mr. Thompson acknowledged that he was a married adult with a college degree who could make his own decisions.

Mr. Thompson also conceded under questioning by Mr. Dreher that he had known it was unlawful to go into the Capitol on Jan. 6 while lawmakers were finalizing the results of the election. That appeared to contradict a central pillar of his own defense.