WASHINGTON — Shortly after 2 p.m. on Thursday, Senator Charles E. Grassley of Iowa administered the oath to Chief Justice John G. Roberts Jr. in the well of the Senate, asking him to swear “that in all things appertaining to the trial of the impeachment of Donald John Trump, president of the United States,” he would deliver impartial justice.

At the White House at that precise moment, President Trump was scheduled to be hosting an event promoting the administration’s attempt to empower religious students to exercise their rights to pray at school. But for the next hour as, one by one, 99 senators signed the oath book, the president kept the students waiting. Finally, he allowed reporters into the Oval Office, where he sat behind the Resolute Desk and took their questions.

The day before, Mr. Trump had presided over a celebration of a trade deal with China, calling out some of the nation’s biggest corporate leaders as if they were his golf buddies. Earlier Thursday, the Senate had given final approval to the United States-Mexico-Canada Agreement. Now he had to talk about the attempt to remove him from office.

“I did the biggest deal ever done in the history of our country yesterday in terms of trade,” he said, referring to (and exaggerating the size of) the China deal, “and that was the second story to a total hoax. Today, we just had passed the U.S.M.C.A. It’s going to take the place of NAFTA, which was a terrible deal, and the U.S.M.C.A. will probably be second to this witch hunt hoax, which hopefully everyone knows is not going anywhere.”