Susan Collins is up for reelection in a state that Hillary Clinton carried in 2016. And although there's nothing to prove it yet in the polling, this race could be very different than the last four, in which she easily swept her Democratic opponents.

Moderation has been Collins’ greatest strength each time. She appeals to voters in both political parties who value thoughtfulness and middle ground. But it’s that very thing that has turned left-leaning voters against her.

Democrats cannot forgive Collins for voting to confirm Supreme Court Justice Brett Kavanaugh. The effect of that vote on her overall popularity was minimal (Morning Consult's polling shows her down all of three points of approval between summer and fall of 2018), but the Left clearly does not care about her reasoning, nor about the dozens of hours she spent going over every single part of Christine Blasey Ford’s accusation against him, nor about the countless closed-door conversations she had with other members of Congress. For some people, moderation is only worth something if it guarantees a specific result.

And because the result was Kavanaugh’s confirmation to the bench, leftist activists spent months harassing Collins and her staffers. In their new book, Justice on Trial, Mollie Hemingway and Carrie Severino detail several alarming encounters. One man accosted Collins outside her Capitol Hill townhouse late at night, shining a flashlight in her face and demanding she answer his questions. A 24-year-old staffer in Collins’s Maine office answered the phone one day to an angry man saying he hoped she would be raped and impregnated. One of Collins’s neighbors called her a “rape apologist.”

Even after the confirmation, Collins and her family continued to face harassment. In October, her husband sent her a picture of himself in full hazmat gear after a letter addressed to him was purported to contain ricin. This was the cost of Collins’ thoughtfulness.

Now Democrats are rallying behind various progressive candidates in an attempt to boot Collins out of office. Maine’s Democratic House Speaker Sara Gideon could be Collins’s most challenging opponent, if she can make it through the primary.

Democrats would have disagreed with the outcome of the Kavanaugh vote regardless of whether Ford had come forward. And Collins was right when she said that most Democrats and special interest groups had made up their mind about Kavanaugh’s guilt before Ford had said even a word against him. They were looking for any weapon with which to kill his nomination, and they found one in Ford. Collins valued Ford’s testimony, but ultimately decided that definite guilt requires more than just one completely uncorroborated accusation (there is still no independent evidence that Ford and Kavanaugh had ever been in the same place at the same time) from high school.

Surely, if Kavanaugh had been a left-leaning judge, the Democrats would have agreed. After all, many of them are already trying to rehabilitate Al Franken, a Democrat who was ousted from his Senate seat based on multiple well corroborated accounts of his very recent sexual harassment, as a grown-up and even as a U.S. senator.

The war against Collins is further proof that in these polarized times, moderation must die at the altar of partisanship. But Congress is a better, more honest place because of Collins. She’s had plenty of disagreements with members of her own party, specifically over social issues and even Obamacare. But Republicans should rally behind her come 2020. The Senate needs independent thinkers such as Collins, and it would set a terrible precedent to let leftist activists bully her into submission.