John R. Bolton was the only topic on the minds of Republicans as they huddled behind closed doors on Monday for a luncheon just before the impeachment trial began. But Senator Mitch McConnell, Republican of Kentucky and the majority leader, urged his colleagues not to rush to any conclusions.

“Let’s get through the next step,” Mr. McConnell said, according to Senator Mike Braun, Republican of Indiana. “Take a deep breath and let’s take one step at a time.”

Mr. McConnell was among those venting angrily to the White House in private on Monday about the leak of the former national security adviser’s manuscript, in which he wrote that Mr. Trump was conditioning the release of military aid to Ukraine upon the country furnishing investigative information about his political opponents. That contradicts the defense that the president’s lawyer’s have offered the Senate, which holds that Mr. Trump never linked the two issues.