Senate Republicans stand to be the biggest casualty of impeachment, since they face losing their three-seat majority as struggling candidates are forced to cast a vote that will anger critical groups of voters, no matter which way they go.

Democrats are targeting President Trump. What they may get instead is the Senate.

In a Senate trial to adjudicate articles of impeachment approved by the House, at least a handful of vulnerable Senate Republicans risk the wrath of grassroots conservatives if they vote to convict and remove Trump from office. The same group, staring down impeachment with the 2020 primary season drawing near, could just as easily alienate general election voters with a vote to acquit the president.

While Trump is unlikely to be convicted by the Senate, some Republicans will be tempted to support it. “The House makes the articles of impeachment; we sit as a jury,” Republican Sen. Chuck Grassley told reporters Monday. “There isn’t any way you’re going to know how that plays out until it’s part of public discussion.”

Republican voters are heavily against impeachment. Democrats are strongly in favor, and independents are leaning supportive.

That dynamic could squeeze Republican incumbents up for reelection in battleground states, forcing them to choose between appeasing a feverishly pro-Trump GOP base or distancing themselves from an unpopular president to appeal to swing voters and soft partisans. Among them are Sens. Cory Gardner of Colorado, Susan Collins of Maine, Thom Tillis of North Carolina, Joni Ernst of Iowa, and Martha McSally of Arizona.

“They were all going to have tough races to begin with, and Trump isn’t making it any easier on them,” said a Republican strategist, who requested anonymity in order to speak candidly.

Under Senate rules, Republicans during a trial would be severely limited from running interference for Trump. Supreme Court Chief Justice John Roberts would preside and hold significant power over motions.

That could present problems for Republicans.

[Read more: McConnell: Trump to face Senate trial if impeached]

Trump and his allies in conservative media have periodically criticized Senate Republicans on Twitter without regard for congressional rules and constitutional constraints, leading the GOP base to expect more from the party than is realistically possible. Especially because impeachment is rare and mysterious to many voters, Republicans could find themselves fending off accusations from the Right that they are abetting the Democrats if the trial goes poorly for the president.

“Senators don’t have an active role. The rules mandate that we sit silently,” Sen. Josh Hawley told the Washington Examiner. The Missouri Republican conceded that there would be virtually nothing he or his GOP colleagues could do to overturn rulings made by Roberts that might damage Trump. “It will be strange and it will be hard,” he said.

As an insurance policy, party insiders are urging Senate Republicans to move swiftly to counter impeachment by using the power that is available to them.

That includes holding committee hearings to investigate the allegations behind the impeachment inquiry and the figures involved in order to undercut the Democrats’ claims that Trump abused his power when he asked Ukrainian President Volodymyr Zelensky to investigate political rival Joe Biden.

“There are things Senate Republicans can do to keep the base happy with them,” said Andy Surabian, a Republican strategist and confidant of Donald Trump Jr., the president’s son.

Impeachment could also clip the Democrats.

Sen. Doug Jones, up for reelection in deep red Alabama, is walking the same tightrope as Republicans who are running for another term in blue and purple states. Ditto Sen. Gary Peters of Michigan, a state Trump narrowly carried in 2016. He faces a stiff challenge from Republican John James.

And with a Senate trial likely to be triggered by a partisan vote of House Democrats, the burden of the prosecuting the case in the Senate chamber, according to the rules, would fall on them. So, too, could the blowback if the public determines that their case is thin and unreasonable.