“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.