For Peter Wehner, who advised President George W. Bush on domestic policy, that is not good enough.

Mr. McConnell and other Republican leaders must “make it clear to Trump and the White House that if he fires Mueller, that is crossing a red line, and there will be consequences,” Mr. Wehner said. One idea: Warn the president that Congress will hire Mr. Mueller to continue his inquiry if Mr. Trump fires him.

“I understand that this is not McConnell’s thing to speak out and challenge people publicly, and maybe he is being extremely effective privately — and if he is, more power to him,” Mr. Wehner said. “But on the other hand, what’s said publicly matters, too. And at some point, if what Trump says and does is met with silence, it implies complicity for the party.”

Mr. McConnell learned the dangers of criticizing Mr. Trump in August, when he was asked in Kentucky about the Senate’s struggle to repeal the Affordable Care Act. Defending the chamber’s work, Mr. McConnell offered an offhand assessment of how the president appeared to be learning on the job, saying Mr. Trump “had excessive expectations about how quickly things happen in the democratic process.”

It was hardly inflammatory, but the comment infuriated Mr. Trump, who responded with a Twitter tirade against the leader that lasted six weeks. The two did not speak for a long stretch during that period, and their relationship deteriorated to the point where Mr. McConnell privately expressed doubts that Mr. Trump could salvage his presidency.

In October, as suddenly as it began, Mr. Trump’s Twitter attack ended. In an orchestrated show of togetherness, he brought Mr. McConnell into the Rose Garden to declare that the two were “closer than ever before.” One person close to Mr. McConnell says he does not want a repeat of what followed the “excessive expectations” remark.

“That caused him six weeks of unrelenting pain of Trump whacking him,” that person said, speaking on the condition of anonymity to discuss Mr. McConnell’s thinking. “I think at that moment, McConnell learned a lesson: There’s nothing to be gained by commenting on his comments because it doesn’t change his behavior.”

Mr. McConnell has always been a man of few words, and one who chooses his words carefully. Unlike other senators who linger in the Capitol corridors to talk with reporters, he shuns hallway interviews and rarely takes more than a few questions at his Tuesday briefings. His reticence is integral to his leadership style.