Sen. Susan Collins hasn’t even announced she’s running for reelection, but she already has a serious Democratic challenger angling to take her down. Last week, Democrat Sara Gideon announced her campaign for Senate in Maine. As speaker of the state House of Representatives, Gideon could be the challenger Democrats need to defeat Collins, who first won her seat in 1996. Maine’s Democratic lean and Collins’s increasingly polarized profile could make the incumbent vulnerable, and Gideon’s entry into the race reflects this.

Experienced candidates are better candidates. Political scientists have found that candidates who have previously held elected office tend to do well in congressional elections because they have qualities that make them more electable, such as strong political skills and connections to donors. But importantly, experienced challengers are also more likely to run when they believe they have at least a decent chance of winning.

And based on Maine’s political makeup, it’s easy to see why Gideon thinks she has a chance of defeating Collins. As the table below shows, Collins holds the most Democratic-leaning seat held by a Republican that’s on the ballot in 2020, based on FiveThirtyEight’s partisan lean metric.

Maine is bluer than Susan Collins Partisan lean for each state currently represented by a Republican senator up for reelection in 2020 Senator State Partisan Lean Susan Collins Maine D+4.9 Cory Gardner Colorado D+1.5 Thom Tillis North Carolina R+5.1 Joni Ernst Iowa R+5.8 Martha McSally† Arizona R+9.3 David Perdue Georgia R+11.8 Dan Sullivan Alaska R+14.9 Cindy Hyde-Smith Mississippi R+15.4 John Cornyn Texas R+16.9 Lindsey Graham South Carolina R+17.2 Bill Cassidy Louisiana R+17.3 Steve Daines Montana R+17.7 Pat Roberts (not running) Kansas R+23.3 Mitch McConnell Kentucky R+23.3 Ben Sasse Nebraska R+24.0 Tom Cotton Arkansas R+24.4 Lamar Alexander (not running) Tennessee R+28.1 Shelley Moore Capito West Virginia R+30.5 Mike Rounds South Dakota R+30.6 James Inhofe Oklahoma R+33.9 James Risch Idaho R+34.9 Mike Enzi (not running) Wyoming R+47.4 Partisan lean is the average difference between how a state or district voted and how the country voted overall, where 2016 presidential election results are weighted 50 percent, 2012 presidential election results are weighted 25 percent, and results from elections for the state legislature are weighted 25 percent. Note that the partisan leans in this table were calculated before the 2018 elections; we haven’t calculated FiveThirtyEight partisan leans that incorporate the midterm results yet. † Special election Source: U.S. Senate

If Maine continues to go blue in 2020, Collins is likely to be in real danger. While the state is not overwhelmingly Democratic, it has consistently voted for the party’s presidential candidate in every race going back to 1992. And nowadays, states usually back the same party for president and Senate.

Collins is also a more polarizing figure now than she once was, especially after her vote to confirm Brett Kavanaugh to the Supreme Court last year. While Collins has built a moderate profile with one of the most centrist voting records in the Senate, her support for Kavanaugh shifted views of her job performance along partisan lines, with sharp increases in approval among Republicans and, conversely, disapproval among Democrats. The former makes it less likely that Collins will get a primary challenge, but the latter suggests Collins could be in trouble in a general election. Gideon’s introductory video specifically referenced the vote, saying it put women’s health choices in “extreme jeopardy.”

While Gideon’s entry probably makes her the leading Democratic Senate candidate, she will have to get through a primary. Betsy Sweet, a progressive who finished third in the 2018 Democratic primary for governor, is already running, and others could join, such as Maine Secretary of State Matt Dunlap. In Maine’s ranked-choice voting system, a large number of competitive candidates could create unpredictable scenarios for winning the party’s nomination. Still, Gideon has had a strong start, receiving endorsements from major Democratic groups, including EMILY’s List, a group that works to elect pro-choice Democratic women to office, as well as NARAL Pro-Choice America and the Democratic Senatorial Campaign Committee, the party’s Senate campaign arm.

But even if Gideon wins the primary, Collins will be a formidable opponent. In March, the first — and so far only — poll testing the Collins-Gideon matchup found Collins leading 51 percent to 29 percent. Gideon’s position will almost certainly improve now that she’s actually in the race — 41 percent had no opinion of her in that survey — but the early gap shows why it’s so hard to challenge an incumbent. Collins has seen her approval slide in recent years, going from 67 percent in early 2017 to 52 percent in the first quarter of 2019, according to Morning Consult, but her standing is much better than Maine’s partisan lean would predict.

Democrats will almost certainly marshal resources to take Collins down. Last year, Collins’s Kavanaugh vote sparked fundraising efforts that so far have collectively raised $4.7 million for the eventual Democratic nominee — sight unseen — in the general election.

Maine’s seat is pivotal to Democrats’ chances of retaking the Senate. Republicans currently have a 53-47 advantage in Congress’s upper chamber, so for the chamber to flip, Democrats need to win three net seats and gain the vice presidency to break a 50-50 tie (or win four seats if President Trump wins reelection). Because Democrats have a fairly limited number of GOP-held seats to target and must defend a seat in heavily Republican Alabama, the GOP likely would retain the Senate if Collins won reelection.

But Gideon may just be the experienced candidate Democrats need to capitalize on Maine’s Democratic lean and Collins’s more polarized image to defeat the incumbent and keep the potential of a Democratic Senate alive in 2020.