Sen. Susan Collins of Maine announced Wednesday that she plans to seek a fifth term in 2020, setting up what is expected to be one of the most expensive and competitive U.S. Senate races in state history.

Collins, a Republican, announced her bid for re-election in a letter to supporters Wednesday, ending months of speculation on whether she would run for another six-year term amid the backdrop of impeachment proceedings and a highly partisan climate.

“I promised the people of Maine a decision this fall on whether I would seek re-election,” Collins said in the letter. “The fundamental question I had to ask myself in making my decision was this: In today’s polarized political environment, is there still a role for a centrist who believes in getting things done through compromise, collegiality, and bipartisanship?

“I have concluded that the answer to this question is ‘yes,’ and I will, therefore, seek the honor of continuing to serve as Maine’s United States Senator.”

The announcement came the same day the House began debate articles of impeachment against President Trump.

Collins has not said how she would vote on impeachment, citing her need to be an impartial “juror” in a possible Senate trial, and said in an interview Wednesday that she will not be surprised if the House approves articles of impeachment.

“I can’t predict how this will play out,” she said. “All I can say is I will approach it as I believe a judge and juror should – in an impartial way and by evaluating all the evidence that is brought to us.”

The race for Collins’ Senate seat is expected to be the most competitive she has faced to date. In each of her previous re-election campaigns, Collins won with at least 58 percent or more of the vote.

“It certainly appears this will be the toughest campaign for Senate she’s ever faced,” said Mark Brewer, a professor of political science at the University of Maine. “You can already see indicators of that with the amount of money coming in, which will expand exponentially. This will be by far the most expensive election campaign in the state of Maine.”

Heading into 2020, four Democrats have announced they are seeking Collins’ seat, including Maine House Speaker Sara Gideon, who raised over $1 million for her campaign within one week of announcing her bid to unseat Collins.

The others are attorney Bre Kidman of Saco, former tech executive and political newcomer Ross LaJeunesse of Biddeford, and longtime progressive advocate Betsy Sweet of Hallowell.

More than $13 million had been raised collectively by the candidates as of Sept. 30, including almost $8.6 million by Collins – more than any candidate in Maine history more than a year before the election.

“Mainers are excited to elect a new senator, and Maine Democrats have been organizing,” Maine Democratic Party Chair Kathleen Marra said in a statement. “With Collins’ support in Maine at an all-time low, we’ve seen volunteer engagement more than double in comparison to the previous off-year and know this excitement will continue to grow.”

Although she is still regarded in Washington as a moderate within an increasingly right-leaning caucus, Collins has lost support among Maine Democrats and some independents, according to several polls. Over the summer, the non-partisan Cook Political Report labeled her seat a toss-up.

Her support of Supreme Court Justice Brett Kavanaugh was widely criticized by progressives and Democrats, and is likely to resurface during the campaign.

The aftermath of the vote turned some women’s and progressive groups against Collins, who supports abortion rights and traditionally has had a strong relationship with Planned Parenthood.

“It hasn’t really come up yet, but I think that’s one thing that stirs the pot for a lot of people on both sides,” said Jim Melcher, a professor of political science at the University of Maine at Farmington. “It’s a thing that some people who used to support her – and now don’t – remember and it’s a thing that for some Republicans who are lukewarm to her might say, ‘Well, at least she was with us on that.'”

Collins said Wednesday she stands by her decision to vote for Kavanaugh, citing a 45-minute speech on the Senate floor in which she said his temperament and past decisions met the qualifications for a Supreme Court justice and there were not enough facts to corroborate sexual assault allegations against him.

“I’ve had people praise me for that vote and speech, and people who are unhappy with it,” Collins said. “I did what I thought was right and thoroughly explained my reasons.”

Collins’ approval ratings from Republicans went up after the Kavanaugh vote, but she also has faced criticism from her own party for not supporting Trump enough and sometimes siding with Democrats, including on a 2017 vote against the repeal of the Affordable Care Act.

Progressive groups and her Democratic opponents were quick to take aim at Collins on Wednesday, linking her to GOP leadership, the Kavanaugh vote and other controversial decisions, such as a $1.5 trillion Republican tax cut bill that she supported.

“When Senator Collins took office 22 years ago, she might have been different from other people in Washington, but it doesn’t seem that way anymore,” Gideon said in a statement. “These days, Senator Collins seems more focused on serving the special interests that fund her campaigns than the Mainers who elected her.”

Kidman noted in an interview that Collins made a pledge early in her career that she would only be a two-term senator.

“Her running again strikes me as an indicator she is not the Susan Collins who first came to the Senate in 1997,” Kidman said.

“Mainers are ready for a U.S. senator who isn’t bought and paid for, regardless of political party,” Sweet said in a statement. “It doesn’t matter how much money corporations and special interests spend in this election, Mainers are ready for a U.S. senator who will fight for big solutions like Medicare For All, a Green New Deal and eliminating student debt.”

“I entered this race because Susan Collins’ loyalty to Maine ends at the door to Mitch McConnell’s office,” LaJeunesse said in an emailed statement. “At a time when … large parts of our state still lack basic infrastructure like high speed internet and mobile phone reception, and tens of thousands of Mainers can’t access affordable healthcare, Susan Collins cast the deciding vote for a $1.5 trillion tax cut that benefits the richest Americans and corporations. After 23 years in the U.S. Senate, Susan Collins has forgotten who sent her there.”

In her letter to supporters, Collins cited her childhood in Aroostook County and the “Maine work ethic” instilled in her as a child. She also touted as accomplishments work with senators across the aisle to repeal a $50 billion tax break for tobacco companies and legislation to guarantee benefits for surviving military spouses.

While advertisements that tout Collins’ bipartisanship have been airing on TV and some of her potential opponents announced their bids six months ago, she held off on making a formal announcement throughout the fall.

Collins said in an interview that she has never announced a campaign earlier than the year in which the election will occur.

“If this had been a normal election cycle I would not have announced until next year,” she said. “I’ve never done a big rally or press conference to announce. I have a full-time job and have been negotiating the appropriations bill. That has been day and night … (but) I did promise people I would make a decision this fall and since fall ends Dec. 21, I put out my statement.”