Republicans say that to President Donald Trump impeachment is an us vs. them war, and he wants everyone on the team. | Alex Wong/Getty Images

Donald Trump made a defiant prediction last week that Senate Republicans will be the bulwark to keep Democrats from ending his presidency.

But behind the scenes, Republicans and Trump’s informal circle of advisers are tempering the president’s bravado, expressing concern over whether the president can truly count on a GOP-led Senate to keep him in office.

While Senate Republicans relish the president’s judicial appointments and many of his domestic policies, they’ve never fully embraced Trump’s style of politics. And that raises eyebrows in Trump world.

“If there’s one place where you can set a trap that nobody expects, it’s the U.S. Senate Republican Conference,” said Michael Caputo, a former Trump 2016 campaign adviser who remains close to the president.

If the House impeaches Trump, the process will be kicked over to the Senate for a trial, in which Trump must lose 20 of the 53 Republican senators to be removed from office.

It’s a prospect that may seem far-fetched. Senate Republican officials say the party still foresees an almost entirely partisan outcome in a potential impeachment trial. And Republicans — especially those up for election in 2020 — have long been afraid of losing Trump’s base if he turns on them. They’ve rarely defected from the president in large numbers for key votes.

But the landscape could shift quickly if fresh details emerge that help Democrats make their case that Trump must go, said Michael Steel, a longtime GOP operative and aide to former House Speaker John Boehner.

“If the Democrats are able to make an argument that’s clear and captures the public imagination … then you start thinking more seriously about what a Senate trial could look like and whether you could get to that possibility,” Steel said of a scenario in which enough GOP senators defect to remove Trump.

For now, Trump and his allies have been using a carrot and stick approach to keep the party in line.

Trump has lobbed attacks at some potential defectors, calling Sen. Mitt Romney a “pompous ass” after the Utah Republican denounced Trump’s public statements seeking help from Ukraine and China to investigate Joe Biden, a top 2020 rival. But he’s sweet-talked others, recently endorsing the reelection campaign of Nebraska Sen. Ben Sasse, one of his most outspoken critics.

Vice President Mike Pence is also glad-handing with senators crucial to an anti-impeachment coalition. On Wednesday, he traveled to Iowa and appeared at an event with Sens. Chuck Grassley and Joni Ernst, who is up for reelection next November. Last week, Pence was in Arizona with Sen. Martha McSally, who is running for a new term in 2020 .

Caught in the spotlight, Senate Republicans are treating their comments on the Democrat-led impeachment inquiry with the utmost care, aware that the president could end their careers. Last year, Trump took down Florida Republican Adam Putnam with a tweet during the state’s gubernatorial primary, putting his weight behind the eventual winner, Ron DeSantis, who took pains to get close to Trump.

Republicans have challenged the president on several major policy issues, like Trump’s decision earlier this week to withdraw U.S. troops from northern Syria, his tariff fights with China and Europe, or his national emergency declaration to circumvent Congress to obtain funds for a border wall.

But Republicans say impeachment is, to Trump, an us vs. them war, and he wants everyone on the team. That’s why numerous senators are struggling to take a stance over whether they believe it’s appropriate for the president to solicit foreign intervention against Trump’s domestic political rivals.

Republican senators “are willing to disagree with the president. But if the fight is obviously driven by excessive partisanship on the other side, that’s a different place to decide how you’re going to engage on that,” said Sen. Roy Blunt (R-Mo.), a member of Senate leadership, in an interview.

“Impeachment, Russian collusion and the Mueller report — all of that has been incredibly partisan in the way they’ve been used by Democrats in the House and the Senate,” he added. “I think Republicans have to take note of that as they figure out how they’re going to respond.”

Many Republicans have yet to decide how to react.

Under intense questioning from CNN in Iowa, Ernst on Wednesday said she didn’t have enough information to determine whether Trump’s actions were appropriate. Maine GOP Sen. Susan Collins has avoided addressing the situation by saying she may have to be a juror at Trump’s trial, though Collins pushed back against Trump’s call for China to probe Biden. On Thursday, Colorado Sen. Cory Gardner repeatedly spun the question into attacks on House Democrats and deferred to the Senate Intelligence Committee.

Democrats are taking notice. In an interview Thursday, aisle-crossing Sen. Chris Coons (D-Del.) said he’d spoken to two Senate Republican colleagues this week who either changed the subject or got off the phone with him when he brought up Ukraine.

Coons said he is no longer sure that the Senate GOP would unanimously let Trump off the hook at a trial, but added that Republicans’ immediate political concern over facing a mean Trump tweet might override all other factors.

“President Trump really goes after anyone who is anything less than completely supportive. The way he’s gone back after Sen. Romney is inappropriate and disgraceful,” Coons said. “If I were a Republican senator, I would make a pretty tough but prompt assessment that history will judge harshly those who do not show some independence from this president.”

Some Republican senators are starting to coalesce behind a wrist-slapping strategy with Trump. What Trump did was not right, they say, but neither is impeachment. It’s movement away from Trump, but not in any way that has Senate Republican officials worried about defections.

The latest Republicans to take that line on Thursday were Sens. John Cornyn of Texas, who is up for reelection, and retiring Sen. Lamar Alexander of Tennessee, who called Trump’s action “inappropriate,” while quickly adding that impeachment is “a mistake.”

“Do I wish President Trump hadn’t raised the issue with the Ukrainian president? Yes,” Cornyn wrote in a fundraising email. “But really, is it right for Democrats to now call for his removal for office over this?”

Other Republican senators willing to admonish Trump pivoted into attacks on House Speaker Nancy Pelosi, House Intelligence Chairman Adam Schiff or the impeachment process itself. Senior Republicans expect many in the party will land on that as the safest approach.

Such a strategy has seemingly paid off for the likes of Sens. Rob Portman of Ohio and Pat Toomey of Pennsylvania, who have avoided any Trump attacks on Twitter despite publicly breaking with the president on his calls for China to investigate Biden. The same goes for Grassley and Ernst, who stood up for whistleblowers amid Trump’s attacks on the official who filed the initial complaint about the phone call in which Trump asked the Ukrainian president to probe the Biden family.

“We can debate judgment, or whether or not other presidents have done it or not … but it’s really a question of whether it’s impeachable,” said Sen. Bill Cassidy (R-La.). “To my mind, it does not pass that sort of impeachable test.”

Given all the tightrope walking, Republicans close to Trump recognize that his acquittal in a Senate impeachment trial shouldn’t be taken for granted.

“There’s most definitely a group of Republican senators who are not originally Trump fans that have endorsed him begrudgingly, who have warmed up to him as their fortunes rise,” said a former senior Trump administration official.

Still other Trump allies acknowledge that while the president could lose a popularity contest in the Senate, that doesn’t mean he’s about to be tossed from the Oval Office.

“Are there some senators who would like to vote to convict Trump? Sure,” said former House Speaker Newt Gingrich. “Are there some people who don’t like Trump? Sure.”

But, he added, there are “very few Republican senators” who are “prepared to say as a historic duty that they believe that the choice of the American people should be repudiated because he’s done something sufficiently bad.”

Republicans close to the White House also say Trump has an ace up his sleeve: Senate Majority Leader Mitch McConnell, who, they reason, has the president’s back. Earlier this week, McConnell attacked the House investigatory process, earning a Trump attaboy on Twitter. McConnell is also running for reelection himself in a state that loves Trump.

“The one person you want more than anyone is Mitch McConnell. He’s a master of the chamber,” said Sean Spicer, the former Trump White House press secretary. “And as long as he’s on your side, you’re OK.”

Sarah Ferris contributed to this report.