“What’s changed Susan?” reads a billboard truck parked outside the Capitol Building. Paid for by a liberal activist group, the billboard is just one more tactic the Democratic Party is using to put pressure on Maine Republican Susan Collins, one of the centrist senators who has indicated support for additional witness testimony during the Senate’s impeachment trial.

The Senate will begin weighing the charges against the president on Tuesday, but Senate Democrats have made it clear that President Trump isn’t the only one on trial: Collins is, too.

Before the House passed the articles of impeachment onto the Senate, the Democrats demanded that Republicans allow them to call additional witnesses during the trial, most notably former national security adviser John Bolton. Senate Majority Leader Mitch McConnell successfully blocked the Democrats’ initial requests, but the chamber will still hold a vote on Tuesday to decide whether witness testimony will be included at all.

There’s a good chance Collins will join the Democrats during the vote. Already, she’s expressed her willingness to work with the other side of the aisle to reach a compromise that will suit both parties. And unlike the other centrist Republicans who have said they’d like to hear from Bolton, Collins has the most to lose. She’s up for reelection in Maine this year, and her Democratic challenger has raised a significant amount of money and has tapped noteworthy endorsements, such as Planned Parenthood’s.

Democrats have already made Collins’s race difficult. The Democratic Senatorial Campaign Committee created a website highlighting her past comments during former President Bill Clinton’s 1999 impeachment and comparing them to her stance today. And her vote for Supreme Court Justice Brett Kavanaugh in 2018 led to an immense mobilization on the Left. Dozens of activist groups have led protests outside her Maine and D.C. offices, and this weekend, protesters at Portland’s Women’s March began to chant, “Hey, hey, ho, ho! Susan Collins has got to go!”

At first, it appeared Collins would prevail despite the Democrats’ efforts to unseat her. Her approval ratings in Maine were still fairly high, and the Republican Party continued to rally behind her. But just last week, Collins was ranked the least-popular senator, with a 52% disapproval rate among Maine voters. This is at least a 10% drop in Collins’s net approval rating from last quarter.

This doesn’t mean Collins has lost. She’s still a strong candidate, and the GOP won’t give up her seat without a fight. But Collins is more vulnerable now than she has ever been, and she will likely be wary of isolating centrist voters who tend to support additional witness testimony during the impeachment trial.

If she does break with her party on the trial rules vote, it’s unlikely the GOP will hold it against her. The Republican Party has much more of a vested interest in maintaining Collins’s Senate seat (a seat the Democrats need if they’re going to flip the Senate in 2020) than they do in preventing Bolton from testifying. And if McConnell can keep some of the other centrist Republicans, such as Sens. Lisa Murkowski and Mitt Romney, from voting in favor of witness testimony, Collins’s vote won’t change anything.

But the Democrats will undoubtedly hold Collins’s vote against her should she stick with McConnell. Their recent antics, such as the anti-Collins billboards and websites, are nothing more than a petty threat. But the Democrats’ accelerated fundraising efforts in Maine are a much different story.