Yes, there are disagreements with the president within his own party. But, for the past two years, Mr. Trump holds too much political power over the Republican base for that dissent to ever gain serious political traction.

Again and again we’ve seen that the Republicans who emerge as Mr. Trump’s loudest critics are often at the end of their political careers: Neither Jeff Flake nor Bob Corker opted to run for re-election to the Senate in 2018; Senator John McCain was nearly 80 years old when Mr. Trump was elected.

In this case, Mr. Burr is the chairman of a committee that prides itself on its bipartisanship. He’s overseeing an investigation that aides say has collected a “mountain” of evidence showing how Russia tried to influence the 2016 election.

But, perhaps most significantly, he announced last year that this would be his final term in office.

Mr. Tillis, meanwhile, is up for re-election in 2020 and, even more important, facing a primary challenge from the right. The Times reported on Thursday evening that he was under intense pressure from associates of the Trumps to rebuke Mr. Burr.

Mr. Tillis explained his thinking to reporters on Capitol Hill on Thursday. “I personally believe Democrats are just trying to keep this thing alive, and it’s their latest launch point to do it,” he said.

When a reporter pointed out that Mr. Burr led the committee that issued the subpoena, Mr. Tillis responded: “I think you’d have to speak to Senator Burr. I stand by my comment.”

Read more: Allies of Trump’s Son Declare War on G.O.P.-Led Senate Panel After Subpoena

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