Sen. Susan Collins (R-Maine) seemed invincible for the past 2½ decades. The longest-serving Republican senator from a traditionally blue state, she’s been reelected three times, each by double-digit margins. She has refused to toe the party line on some big issues — from health care to Amy Coney Barrett’s Supreme Court confirmation — and, unlike many other GOP senators, lived to tell the tale.

But in 2020, Collins looks deeply vulnerable for reasons that say a great deal about how her party, and American voters as a whole, are changing.

The numbers are stark: According to the RealClearPolitics average of polls conducted in Maine, Collins trails Democratic challenger Sara Gideon by four percentage points, and the latest fundraising reports show Gideon strongly outraising Collins in their 2020 race. Worse yet, 45 percent of Mainers viewed Collins negatively in a September Suffolk University poll.

Two trends have damaged Collins. In the short term, the decline of ticket-splitting has hurt her chances at reelection. In the long term, the GOP’s turn from patrician moderation to populism is squeezing Collins — and many others like her — out of the party.

Collins’s most immediate problem is that voters have become less willing to differentiate her from her party.

During her early campaigns, Collins won by convincing liberal and moderate voters that she was smart, bipartisan and worth voting for — even when the national GOP was in bad shape. She ran as a business friendly moderate and won in 1996, despite Republican Bob Dole’s trouncing in the presidential race. And she was the only blue-state Republican to successfully distance herself from unpopular then-President George W. Bush and hold on to her seat in the Democratic wave of 2008.

But since then, voters have become less likely to separate any down-ballot politician from their party’s presidential candidate. As Republicans and Democrats in Washington move further away from one another — both on policy and cultural affectations — many rank-and-file voters have become less tolerant of legislators who yield any ground to the opposing tribe. In many red states, Republican voters lost patience with their centrist Democratic representatives, exchanging moderates for reliable Republican senators in Arkansas, North Dakota, South Dakota, Montana, Alaska, Louisiana, West Virginia, Missouri and Indiana. In the 2018 House elections, Democratic voters returned the favor by picking off some of the last blue districts held by the GOP.

This growing bipartisan intolerance for bipartisanship now threatens Collins. Sure, some voters give her credit for opposing President Trump’s priorities on roughly one-third of all Senate votes, especially her decision to protect the Affordable Care Act from GOP “repeal and replace” attempts and her “no” vote Monday on Barrett, who Collins maintains was nominated too close to the election. But many Biden voters know that Collins is a Republican who ultimately sided with the Trump administration on high-income tax cuts, Brett M. Kavanaugh’s appointment to the Supreme Court and Trump’s impeachment. In this polarized political climate, that’s enough for Collins to lose their support, no matter what else the senator might do.

Even if Collins does eke out a win in November, she’ll be haunted by a longer-term problem: The GOP is no longer interested in her style of moderation.

The Trumpian GOP has a simple formula for winning — or at least trying to win — elections: deliver policy victories for social conservatives and immigration restrictionists, remain skeptical of ideas like the concept of systemic racism while trumpeting support for law enforcement and the military, and hope that economic success delivers swing voters. That pitch is designed not just for campaign season, but to transition the GOP from being a socially conservative, business-friendly party into a socially conservative, nationalist party.

That version of the GOP would leave Collins homeless. Historically, she had the most in common with the party’s moderate, business-minded wing — voters that Trump traded away for the white working class in 2016. She can still temporarily win primaries by supporting the administration on key issues. But broadly speaking, GOP primary voters don’t want moderates like her. If she makes it through the 2020 general election, those preferences might catch up with her in 2026.

Collins is stuck. The newly populist GOP will keep forcing her into hard votes. And Collins will have to either fall in line — or else risk a primary challenge that she could not overcome.

If Collins were a different kind of moderate — maybe, as others have suggested, an economically liberal immigration restrictionist who could better appeal to both moderates and Trumpian Republicans — she might have a better chance in the GOP’s brave new realignment. But chances are that sometime, this year or a bit down the road, she’ll become another casualty of our realigning parties.

Read more: