Sara Gideon, the speaker of the Maine House of Representatives, said on Monday that she would challenge Senator Susan Collins, making her the most formidable opponent yet for a United States Senate seat that Democrats have identified as a target in the 2020 election.

Ms. Gideon, a Democrat, said that her decision was spurred partly by the vote that Ms. Collins, a Republican expected to seek her fifth term, cast in support for Justice Brett M. Kavanaugh’s confirmation to the Supreme Court.

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Ms. Collins, 66, had gained a reputation for independence and bipartisanship during 22 years in the Senate — a fact acknowledged by Ms. Gideon in her announcement video. More recently, though, she has voted repeatedly to support President Trump’s agenda, in what has been perceived as a veer to the right. Her approval rating has also declined in some public polls.

“At one point maybe she was different than some of the folks in Washington,” Ms. Gideon said in the video. “But she doesn’t seem that way anymore.”