Washington DC - Independent Senator Kyrsten Sinema said Tuesday she will not run for re-election in Arizona, a swing state seen as a must-win for control of the finely balanced US Senate in November's election.

The former Democrat, who moved to the upper chamber from the House of Representatives in 2019 and later quit the party, would likely have failed to win a second term, as she consistently polled third behind the main party candidates.

Republicans need a net gain of two seats to win the Senate, but polling is sparse in Arizona's narrow contest, and it is not clear whether Democratic candidate Ruben Gallego or Republican hopeful Kari Lake will benefit most from Sinema's withdrawal.

"Because I choose civility, understanding, listening, working together to get stuff done, I will leave the Senate at the end of this year," the 47-year-old from Arizona said in a video posted on X.

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The former social worker, who left the Democrats to serve as an independent in December 2022, had set out a strategy for November's election of winning up to 20% of Democrats and 35% of Republicans.

Sinema cultivated a reputation as a centrist and a bipartisan dealmaker on contentious legislation, although as a Democrat, she was as often a thorn in the side of the White House, refusing to toe the line on multiple key issues.

"I believe in my approach, but it's not what America wants right now," she said.