Sen. Susan Collins (R-ME) speaks to reporters as she arrives for a nomination vote on Capitol Hill in Washington, December 13, 2017.

Democratic Maine House Speaker Sara Gideon will run for U.S. Senate, giving endangered Republican Sen. Susan Collins her most notable challenger yet in a growing field.

Gideon, 47, announced her candidacy Monday morning. In a video, she touted efforts to work with the GOP in Maine and targeted Collins for backing the 2017 Republican tax law and voting to confirm Supreme Court Justice Brett Kavanaugh.

"Susan Collins has been in the Senate for 22 years, and at one point maybe she was different than some of the other folks in Washington. But she doesn't seem that way anymore," Gideon said.

I'm running against Susan Collins for U.S. Senate because Mainers deserve a senator who will always put our state first. Let's build this campaign together. Will you join us? https://saragideon.com/ #MESen #MEpolitics

While Gideon appears to have support from national Democrats, she still has to get through a Democratic primary to face Collins. Liberal activist and statehouse lobbyist Betsy Sweet — who finished third in the Maine gubernatorial primary last year — and lawyer Bre Kidman will also run for Senate as Democrats.

Despite Democrats' recent success in statewide Maine elections, beating Collins could prove daunting for Democrats. The senator, who has served since 1997, has tried to craft a bipartisan reputation in Congress and has not run in a close race in more than 20 years.

In a statement Monday, Collins campaign spokesman Kevin Kelley said the senator "will continue to build on her record of extraordinary accomplishments for the people of Maine." He noted that Democrats will have to run in a primary for more than a year — which can lead to a nominee emerging hampered.

The GOP appears set to promote Collins as an independent voice in the Senate. In a separate statement, National Republican Senatorial Committee spokesman Nathan Brand touted "her ability to work across party lines to get things done for Maine."

Republicans currently hold a 53-47 majority in the Senate, which has allowed Senate Majority Leader Mitch McConnell to confirm President Donald Trump's judicial and executive branch nominees while stifling nearly all of the legislation that comes out of the Democratic-held House. Democrats see Collins as one of their top targets as they try to flip four GOP-held seats and gain control of the chamber.

Gideon has served in the Maine House since 2013 and became speaker in 2016. Her party sees positive signs in how the state has voted in recent years.

In the 2016 presidential contest, Hillary Clinton defeated Trump by about 3 percentage points in Maine. Last year, Democratic Gov. Janet Mills won by more than 7 percentage points. Independent Sen. Angus King, who caucuses with Democrats, won by nearly 20 percentage points.

The 66-year-old Collins heads into the 2020 election cycle facing more furor than she ever has in her career. Democrats — who worry Kavanaugh's confirmation threatens abortion rights — have slammed Collins for her vote last year to put the justice on the top U.S. court.

She also angered conservatives in 2017 by helping to sink a Republican plan to dismantle the Affordable Care Act. Her reputation as a center-right lawmaker has already earned her a primary challenge from conservative pro-Trump blogger Derek Levasseur.

Collins' popularity in the state has suffered during the Trump administration. In the first quarter of 2017, 67% of Maine voters approved of the job she was doing, while only 27% disapproved, according to Morning Consult polling. This year, 52% of voters approved of Collins' work in the Senate, while 39% disapproved.

Still, she remains more popular than many of her Republican and Democratic colleagues facing tough reelection bids next year.

Collins also has the money to withstand a tough challenge: her campaign ended the first quarter with $3.8 million on hand.

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