Independent US Senator Kyrsten Sinema of Arizona said she will not run for a second term, ending prospects for a turbulent three-way race in one of the nation’s most politically competitive states.

Sinema, who switched her party affiliation from Democrat to independent in 2022, criticised partisan dysfunction in Congress in announcing her much-anticipated decision.

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

Sinema, 47, recently helped clinch a failed border deal with Republican senators that would have cracked down on illegal border crossings, make it harder to apply for asylum and speed up deportations of undocumented migrants. Republicans repudiated the deal after Donald Trump criticised it.

Her decision follows anaemic fundraising during the final three months of 2023, with a US$535,000 haul that was a fraction of the amounts raised by Republican firebrand Kari Lake and progressive Democratic congressman Ruben Gallego, the favourites for their parties’ nominations.

Sinema usually votes with Democrats, which has given the party a 51-seat majority and greater ability to confirm judicial nominees.

The announcement sets up a race between Lake, who refused to concede her loss in Arizona’s 2022 gubernatorial race, and Gallego, a military veteran who has been in the House since 2015.

An Emerson College Polling/The Hill survey conducted last month showed Gallego maintaining and possibly enlarging his lead in a two-person contest against Lake. Gallego led by 7 percentage points in a hypothetical two-way contest, up from 6 points in a three-way race. The margin of error was 3 points.

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Gallego applauded Sinema’s years of public service in a response to her announcement, saying he would welcome her to “join” his efforts in the coming election.

“Protecting abortion access, tackling housing affordability, securing our water supply, defending our democracy – all of this and more is on the line, Gallego said.

Minutes after her announcement, Gallego’s campaign sent out a fundraising appeal declaring the Senate contest a two-person race.

Lake made her own appeal to win the votes of Sinema’s supporters soon after the announcement.

“Senator Sinema had the courage to stand tall against the Far-Left in defence of the filibuster – despite the overwhelming pressure from the radicals in her party like Ruben Gallego who called on her to burn it all down,” she said on the social media platform X.

Arizona Republican US Senate candidate Kari Lake takes questions at a news conference in Phoenix, Arizona, in February. Photo: AFP

Arizona’s voter registration is nearly evenly split between Republicans, Democrats and independents, although Democrats are slightly outnumbered.

Sinema infuriated many Arizona Democrats and progressives nationally by defending the filibuster and opposing a US$15-per-hour minimum wage. Sinema forced Democrats to drop from their signature climate package a “carried-interest loophole” that would have imposed more taxes on private equity managers.

She was censured by the state Democratic Party in January 2022 over her refusal to do away with the filibuster to force through voting rights legislation.

But she helped negotiate a string of 2022 bipartisan victories, including Biden’s bipartisan infrastructure law, legislation protecting same-sex marriage and the first significant gun control measure in many years.