Some of Lee’s colleagues have suggested it was inappropriate for Trump to urge the Ukrainian president to investigate Joe Biden. The second-term senator argues that the president is squeaky clean, and that unless new information comes out at the trial, he already knows how he will vote on charges Trump abused his office and obstructed a congressional investigation.

“What he did was not impeachable. It was not criminal,” Lee said. “And I don't think what he did was even wrong.”

Lee opposed Trump’s run for president in 2016 and even fought his nomination at the GOP convention. Lee also tangled with Trump over his emergency declaration for the border wall earlier this year.

But now, Lee has emerged as one of the de facto leaders of the case to acquit the president. It’s the latest example of the libertarian-leaning conservative’s continued evolution as a senator — from a hard-liner eager to buck leadership to a powerful ally of a GOP president he once shunned.

Once known as a key instigator in the failed 2013 fight to defund Obamacare, Lee has become an integral cog in the Senate GOP machinery, at least when it comes to impeachment. He’s met with McConnell about impeachment strategy and praises the posture of a GOP leader he once clashed with regularly.

Republicans who have fought with Lee in previous instances are delighted to have him as an ally at such a critical moment.

“He’s offended by the process in the House and he wants to make sure the Senate trial is not turned into a circus,” said Graham, who has clashed with Lee on foreign policy and immigration. He’s a senator “we all respect and the White House sought out his views … he’s been very constructive."

Lee's work under the hood of the Senate impeachment trial is not exactly something he's advertising: Several GOP senators said they were unaware of the great lengths the Utahn has gone to prepare the White House for the historic trial, and colleagues said he's relatively reserved at party lunches.

Lee's effort comes amid harsh criticism from Democrats that Republicans are working too closely with the White House on the trial. But few are surprised that Lee is one of the ringleaders.

“That sounds like Mike,” said Sen. Mazie Hirono (D-Hawaii), who serves on the Judiciary Committee with Lee. “I expect all the Republicans around here to be totally coordinating with the White House because they don’t seem to have an independent mind.”

Lee isn’t falling in line on everything. He pushed a criminal justice reform bill over McConnell’s objections last year and thrashed this month’s spending package signed by Trump as the “worst Christmas movie" ever. He also helped lead the Senate’s campaign to curtail the United States’ role in the Saudi-backed war in Yemen — an effort that was opposed by Republican leaders and which Trump was forced to veto.

But Lee has also been there for the GOP when it counts. His work on impeachment echoes his successful bid to persuade Sen. Jeff Flake (R-Ariz.) to support Supreme Court Justice Brett Kavanaugh. Both episodes show a conservative rabble-rouser who is willing to be an effective team player on some of the most difficult issues of the day.

And notably, Lee is perhaps one of the few people in the GOP caucus who can win praise from both Trump-skeptic Republicans like Romney and senior officials in the White House.