Sen. Susan Collins officially launched her bid for a re-election Wednesday, setting up an expensive and closely watched battle for the seat the moderate Republican from Maine has held for nearly 24 years.

Collins made her formal announcement in an email to supporters, saying her "bipartisan commonsense approach" has been key to many legislative successes and will be important in an era of bitter partisanship.

"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," Collins said.

Collins' campaign for a fifth term could be her most difficult race yet and is projected to be the most expensive political race in Maine history. The 66-year-old centrist is viewed as freshly vulnerable in a state where a tradition of political independence is clashing with rising polarization and partisanship.

Democrats have targeted Collins for her votes for Supreme Court Justice Brett Kavanaugh and the GOP tax cut. They have also sought to link her to President Donald Trump and his brand of brash, divisive politics, and have accused her of failing to do enough to stand up to him. Trump is reviled in the state's populous south, anchored by liberal Portland, but cheered in the rural north.

Collins, who says she didn't vote for Trump in 2016, is likely to face a dramatic vote on whether to convict the president in an impeachment trial in the Senate, something that will anger either Democrats or Republicans.

First elected in 1996, Collins has practiced a measured, moderate brand of politics aligned with the ethos of a state where unenrolled voters comprise the biggest voting bloc. Her popularity has held even as she remains the last New England Republican in Congress.

She has shown her independence by trying to distance herself from Trump. She criticized his emergency declaration to build a wall at the southern border, stood up for the anonymous whistleblower under attack by Trump, and criticized the president's withdrawal of troops from Syria.

On impeachment, she has avoided weighing in by noting that she could become a juror in a trial in the Senate.

Four Democrats vying for the party's nomination to face her include Maine House Speaker Sara Gideon, who is backed by the Democratic Senatorial Campaign Committee. The others are activist Betsy Sweet, attorney Bre Kidman and former Google executive Ross LaJeunesse.

Gideon raised $1 million more than Collins in the most recent reporting cycle. But Collins has raised more than double Gideon's amount — $8.6 million — the largest of any political candidate in Maine history.

The Democratic nominee is also expected to benefit a crowdsourced nest egg topping $4 million. With money pouring into the race from dark-money groups, pundits suggest upward of $80 million to $100 million could be spent on this race before Election Day 2020.