Mr. Trump made his comments while standing alongside Colombia’s president, Juan Manuel Santos. As he dug in his heels, Mr. Trump further muddied his explanation of why he dismissed the F.B.I. director, James B. Comey — and in doing so, offered a different version of events from one given to senators just hours earlier by the deputy attorney general, Rod J. Rosenstein.
In that meeting, Mr. Rosenstein outlined his decision to appoint Robert S. Mueller III, a former F.B.I. director, as special counsel. Several senators emerged afterward to say Mr. Rosenstein had told them he was certain that the president was going to dismiss Mr. Comey, even before he wrote a memo critical of the director’s performance.
But at his news conference, Mr. Trump reverted to the White House’s original claim that he was primarily responding to Mr. Rosenstein’s recommendation to dismiss Mr. Comey. The president had later claimed that he had moved against Mr. Comey in part because of his frustration over the F.B.I.’s handling of the Russia investigation.
“Director Comey was very unpopular with most people,” Mr. Trump said on Thursday. “When I made that decision, I actually thought that it would be a bipartisan decision. Because you look at all of the people on the Democratic side, not only the Republican side, that were saying such terrible things about Director Comey.”
There was a palpable sense among the senators who filed out of the briefing room that the center of gravity in the investigation was shifting from Capitol Hill to Mr. Mueller, who will spend weeks assembling a staff and a list of witnesses to interview.
“This pretty much shuts Congress down,” Senator Lindsey Graham, Republican of South Carolina, told reporters. “Democrats, you got what you wanted. You got a special counsel. Now we’ll just move on. We’re not prosecutors.”
The prospect of Mr. Mueller, a former federal prosecutor with a reputation for moral rectitude and an exacting management style, opening a wide-ranging investigation further rattled a demoralized White House staff, only a day before Mr. Trump was scheduled to depart on a grueling nine-day trip to the Middle East and Europe. Several of Mr. Trump’s advisers and associates have urged him to hire an experienced outside lawyer to help him deal with the surging controversy over whether his campaign had ties to Russia, according to several people briefed on the conversations.
Mr. Trump began the day in a defiant mood, abandoning the conciliatory tone the White House had shown in a statement Wednesday evening. In an early-morning Twitter post, he cited, without evidence, “illegal acts” committed by the administration of his predecessor, President Barack Obama, and the campaign of his former opponent, Hillary Clinton — and said they had never led to a…