US Senator Kyrsten Sinema has announced her intention to leave the Democratic Party, four years after being elected.
But she emphasised that she will not sit with Republicans and said she would instead serve as an independent member in the chamber.
The Democrats maintain control of the upper chamber with Vice-President Kamala Harris’s tie-breaking vote.
The Arizona lawmaker said she made the move to fight what she called a “broken partisan system”.
“Registering as an independent and showing up to work with the title independent is a reflection of who I’ve always been, and it’s a reflection of who Arizona is,” she said in a Twitter video.
“We don’t line up to do what we’re told, we do what’s right for our state and our country.”
In an opinion piece written for the Arizona Republic newspaper, she said that “Americans are increasingly left behind by national parties’ rigid partisanship” and that she hoped to “work proudly with senators in both parties”.
She joins Bernie Sanders of Vermont and Angus King of Maine – who both sit with the Democratic party – as the chamber’s only independent senators.
The 46-year-old did not address whether she will run for re-election in 2024, and informed Democratic Leader Chuck Schumer of her decision on Thursday. US media also reported that she had informed the White House of her intention.
Practically, the decision might make little difference to the Senate after Democratic Senator Rafael Warnock’s win in Georgia earlier this week gave the party a 51-49 majority in the chamber.
“We understand that her decision to register as an independent in Arizona does not change the new Democratic majority control of the Senate,” White House Press Secretary Karine Jean-Pierre said in a statement. “We have every reason to expect that we will continue to work successfully with her.”
While the White House statement described her as a “key partner” in many of President Joe Biden’s legislative successes, Senator Sinema has long been willing to vote against the party line on a variety of issues, and some of her actions have angered local party officials in Arizona.
She was the last Democrat to hold out against Mr Biden’s $700bn (£577bn) climate and tax bill in August.
And her refusal to abandon the filibuster – a mechanism rule which requires 60% majority to pass certain legislation in the Senate – infuriated some colleagues earlier this year.
Democratic Senate Majority Leader Chuck Schumer said she will be allowed to keep her Senate committee positions in a statement on Friday.
“Kyrsten is independent; that’s how she’s always been,” the New York Democrat said, adding that he looks forward to continuing to work with her.
Even before her switch she had faced rumours of a primary challenge in 2024 from Congressman Ruben Gallego.
In recent years the Republican party has lost its once vice-like grip over Arizona politics. Senator Sinema’s victory over Martha McSally in 2018 was the first of a wave of Democratic Party victories.
Since then, former astronaut Mark Kelly has taken the state’s other senate seat, while Katie Hobbs was elected governor in last month’s mid-term elections.
Since she was first elected to the Senate in 2018, Kyrsten Sinema has steadily moved into her own political orbit, independent from the gravity of the Democratic Party. That separation, obvious but unstated for years, is now official.
The immediate implications for the US Senate, where Democrats just secured an outright 51-49 majority, are unclear. If Ms Sinema works with the Democrats, which she says she will, the committee power that comes with that newfound majority will remain. Expect Republicans, however, to try to convince her to change her mind.
More concerning for Democrats is when Ms Sinema’s Arizona Senate term expires in 2024. Given the way she has announced her independence, it appears Ms Sinema still eyes re-election. It had seemed increasingly probable that she was going to face a stiff challenge for the Democratic nomination in the state. Now there is the potential for a three-way general-election race pitting her against a Democratic candidate and a Republican.
While Arizona has shown an affinity for iconoclastic politicians in the past, winning as an independent without the support of one of the two major US parties is a difficult task.