Sometimes they are in bitter fights with the president, challenging his nationalist policy approach as an affront to traditional conservatism while also questioning his mental fitness for office. Other times they drift into a deep public swoon for the chief executive that seems to directly contradict their previous criticisms.
The latest shift, just ahead of Tuesday's address, came from Sen. Bob Corker (R-Tenn.), who, after he decided not to run for reelection, engaged in a heated public dispute with Trump over his temperament and "the lack of desire to be competent on issues."
"Nothing has changed," Corker lamented in one October interview on CNN.
Flash forward to this week, and Corker, who just returned from the World Economic Forum in Switzerland, is aggressively touting Trump's performance there, as well as his overall standing on the world stage.
What changed? World leaders' chatter about the Republican tax cuts signed into law last month registered with Corker.
"The one takeaway was how big a deal the tax reform package was — it was unbelievable to me. I just was shocked by that," said Corker, who initially opposed the plan in the Senate before voting for it.
"It really caused the audience there, the people there, to be far warmer to the president, because of this package. It was just an amazing thing," he said in a Capitol interview on the eve of Trump's speech.
Democrats had said they expected Trump's speech to be a sugar high — a well-composed address read from a teleprompter that would hit on enough good topics but lack staying power because of the likelihood that the president would return to his freewheeling ways within a day or two.
"This is a moment. I think he'll get a boost that's temporary," said Rep. Peter Welch (D-Vt.).
Corker is not the only Republican who shifts on the subject of Trump. Sen. Lindsey O. Graham (R-S.C.) has retreated to a relatively neutral posture, though he at times can be outright hostile.
Back in 2016, Graham endured Trump's ridicule during his crash-and-burn bid for the GOP nomination, warning that a Trump presidency would spell the end of the Republican Party. He linked arms with Democrats in calling for an aggressive, independent investigation of any connections between Trump and Russia.
But in the fall and early winter, Graham emerged as a Trump ally, playing golf with the president and trying to broker deals on everything from health care to immigration. It confounded Graham's friends and enemies alike.
Then came a pair of White House meetings, two days apart, earlier this month. One, on a Tuesday, saw Trump commanding a freewheeling discussion for 55 minutes over how to handle undocumented immigrants who were brought here illegally as children. Sometimes Trump's comments seemed at odds with his party's own positions on immigration, but it was mostly a solid performance.
Two days later, in a small huddle with four senators, Trump and Graham got into a shouting match about immigrants from some African nations after the president had said they were from "shithole" countries.
Ever since, Graham has suggested that Trump has something close to a split personality: "Tuesday Trump," the one who oversaw the public meeting, and "Thursday Trump," the angry nativist.
How did Graham expect Trump would do? "If it's the Tuesday Trump, it's going to be a home run; the Thursday Trump, not so much," Graham said. "Well, it's on Tuesday, that's the good news."
Most presidents would follow the prime-time televised speech by immediately going out into the country to do a few rallies on the main topic, trying to build support for the policy outlines of the address.
Trump has no such plans, and that was fine with Graham. The senator was unsure which Trump would show up at those rallies — the Tuesday or Thursday version — and he said it's probably better to just let the media keep reporting about corporations giving bonuses and better benefits to workers rather than risk a presidential rally that could veer off course.
"There's an argument to be made to get out of a good story's way, just let it breathe," Graham said.
Democrats are furious that most Republicans have not been more critical of Trump's actions, including Republicans on the House Intelligence Committee who voted Monday to release a partisan memo criticizing the FBI's handling of the investigation into the Trump campaign's potential connections to Russian meddling in the 2016 elections.
"It's a breakdown of the norms," Welch said.
At Tuesday's weekly House Republican leadership news conference, Speaker Paul D. Ryan (R-Wis.) engaged in his own back-and-forth on Trump matters. He said he hoped the presidential address would focus on the improving economy — and then every question he received dealt with the various investigations into Russian matters.
Ryan defended the release of the GOP's memo because of possible "malfeasance" by the FBI, giving credence to the idea in some circles that Deputy Attorney General Rod J. Rosenstein has not done a good job overseeing the investigation by special counsel Robert S. Mueller III.
Yet, less than a minute later, the speaker went out of his way to praise Rosenstein, saying there is "no reason" to dismiss him.
With so much noise, it's difficult to know whether one well-delivered speech can put Trump on the right track.
"I thought his speech in Davos was one that changed the arc," Corker said.
Graham was less optimistic. "The Tuesday Trump, seven days a week," Graham said. "Yeah, it is possible, you've just got to have discipline."
The same could be said for Republicans on Capitol Hill.