A month and a half ago, the pollster Critical Insights released a poll that said Republican Sen. Susan Collins’s approval rating had dropped to 41 percent and her disapproval rating had risen to 42 percent. But it was just one poll, and others found that she still enjoyed strong support.

However, Morning Consult’s quarterly poll of senators’ approval ratings, released on Wednesday night, agrees that Collins’s popularity is underwater. Only 45 percent of Mainers said they approved of her compared with 48 percent who said they disapproved, and only one senator (Majority Leader Mitch McConnell) has a higher disapproval rating than Collins now does.

This is a stark turnaround for Collins, who was once one of the most popular senators in the country. In the first quarter of 2017, for instance, Morning Consult gave her a +40 net approval rating (her approval rating of 67 percent minus her disapproval rating of 27 percent). But her reputation for bipartisanship took a hit last fall, when she voted to confirm Brett Kavanaugh, President Trump’s Supreme Court nominee who faced multiple allegations of sexual misconduct. Granted, her popularity was already on the decline before that vote (she had a +22 net approval rating in the second quarter of 2018), and it didn’t nose-dive immediately after it (it was still +15 in the fourth quarter of 2018), but importantly, it does seem to have polarized her support along party lines.

Before the vote, Morning Consult found Collins enjoyed the approval of most Democrats and independents, while Republicans were split 47-47 on her job performance. But after the vote, her net approval among Republicans surged, while it dropped with independents and plummeted among Democrats. In the latest poll, her net approval rating was +27 among Republicans, -2 among independents and -40 among Democrats.

That’s a bad trade for Collins, because Maine is a Democratic-leaning state (it has a FiveThirtyEight partisan lean of D+5), so she will need the support of some independent and/or Democratic voters to win reelection in 2020. With these latest numbers from Morning Consult, Collins has a Popularity Above Replacement Senator score of +2 — in other words, she is just 2 points more popular than we’d expect a generic Republican to be in Maine. Just three months ago, her PARS score was +18. That personal popularity was supposed to go a long way toward helping her get reelected against the partisan tide. But now her popularity simply falls along party lines, just like it does for fellow vulnerable Republican Sens. Cory Gardner (a +1 PARS), Joni Ernst (-1) and Martha McSally (-6), who are also up for reelection in 2020. With the decline of split-ticket voting for races at the top of the ballot, that could mean Collins is in real jeopardy of losing if the Democratic presidential nominee wins Maine in 2020.

And unlike her 2008 and 2014 reelection campaigns, Collins was already facing a tough race in 2020. In the aftermath of her vote on Kavanaugh, progressive groups collectively have raised $4.7 million for her eventual Democratic opponent. And in recent weeks, multiple Democrats announced they would run for her seat, most prominently state House Speaker Sara Gideon, who was quickly endorsed by the Democratic Senatorial Campaign Committee and raised $1.1 million in her first week. Together with this poll, these latest developments in the race support the idea that Collins may turn out to be of the most vulnerable senators up in 2020.