Senator Kyrsten Sinema of Arizona, the Democrat turned independent who cut bipartisan deals that cemented parts of President Biden’s agenda but also stymied some of her former party’s highest priorities, said on Tuesday that she would not seek re-election.

Her announcement ended a year of speculation about her future in a politically competitive state. It cleared the field for a traditional matchup in the high-stakes battle for control of the Senate, between a more conventional Democrat, Representative Ruben Gallego, and the eventual Republican nominee.

“Because I choose civility, understanding, listening, working together to get stuff done, I will leave the Senate at the end of this year,” she said in a video announcement.

Ms. Sinema, a first-term senator who left the Democratic Party in 2022, faced potentially long odds in a three-way race for re-election as Democrats fight to maintain control of the Senate. Recent polling found her trailing both Mr. Gallego and Kari Lake, the favorite for the Republican nomination who is an acolyte of former President Donald J. Trump and has championed his baseless election fraud claims.

Her decision to bow out now set up a more direct showdown, likely between Mr. Gallego and Ms. Lake — though Mark Lamb, a sheriff, is also seeking the Republican nomination.

Senator Steve Daines of Montana, who heads Republicans’ Senate campaign committee, argued that Ms. Sinema’s decision not to run would boost Ms. Lake, who is endorsed by his National Republican Senatorial Committee.