
Independent Senator Kyrsten Sinema of Arizona has announced she will not run for re-election in November.
She decided to leave the Senate "because I choose civility, understanding, listening, working together on stuff", she said.
Ms Sinema switched from being a Democrat to an independent in 2022 but typically votes with her former party.
This sets up a straight fight between Republicans and Democrats, who narrowly control the upper chamber.
It is unclear which party benefits more from her announcement.
While her departure means the Democrats lose a senator who votes with their party, some Democratic strategists fear a three-way fight in November.