WASHINGTON — Democratic Sen. Kyrsten Sinema of Arizona announced Friday she has registered as an independent, a renegade move that could bolster her political brand but won’t upend the Democrats’ narrow Senate majority. She says she will not caucus with Republicans.

Sinema, who faces reelection in 2024, has been a vibrant yet often unpredictable force in the Senate, tending toward the state’s independent streak and frustrating Democratic colleagues at times with her overtures to Republicans and opposition to Democratic priorities.

Sen. Kyrsten Sinema, D-Ariz., speaks during a Senate Homeland Security and Governmental Affairs committee hearing Sept. 14, on Capitol Hill in Washington. Alex Brandon/Associated Press, file

“I just don’t fit well into a traditional party system,” Sinema she said in an interview Friday.

In the interview, Sinema said she hasn’t decided whether she will run for reelection. But she said this was the time to be “true to myself and true to the values of the Arizonans I represent.”

“I don’t expect anything to change for me,” she said. “This will just be a further affirmation of my style of working across all the political boundaries with anyone to try and get something done.”

While unusual for a sitting senator to switch party affiliation, Sinema’s decision may well have more impact on her own political livelihood than the operations of the Senate. She plans to continue her committee positions through the Democrats. Her move comes just days after Democrats had expanded their majority to 51-49 for the new year, following the party’s runoff election win in Georgia.

In a statement, Senate Majority Leader Chuck Schumer said Sinema had informed him of her decision and asked to keep her committee assignments – effectively keeping her in the Democratic fold.

“Kyrsten is independent; that’s how she’s always been,” Schumer said. “I believe she’s a good and effective senator and am looking forward to a productive session in the new Democratic majority Senate.”

The Democrats “will maintain our new majority on committees, exercise our subpoena power and be able to clear nominees without discharge votes,” he said.

In case of tie votes, Vice President Kamala Harris will continue to provide the winning vote for Democrats.

Sinema, who has modeled her political approach on the maverick style of the late Republican Sen. John McCain of Arizona, will join a small but influential group of independent senators aligned with the Democrats – Sen. Angus King of Maine and Sen. Bernie Sanders of Vermont.

At the White House, press secretary Karine Jean-Pierre praised Sinema as a “key partner” in passing some of President Biden’s priorities and said the switch “does not change the new Democratic majority control of the Senate. … We have every reason to expect that we will continue to work successfully with her.” Sinema informed the White House on Thursday afternoon about her plans to formally leave the Democratic Party, according to a person familiar with the discussion and granted anonymity to disclose a private conversation.

Sinema has been at the center of many deals brokered during this session of Congress – from a big, bipartisan infrastructure package Biden signed into law to the landmark bill approved this week to legally protect same-sex marriages.

The move to forgo a political party will scramble the Senate election landscape for 2024 as Democrats already face a tough path to maintaining Senate control. Her switch risks splitting the Democratic vote in Arizona between her and the eventual Democratic nominee, giving Republicans a solid opening.

A splintered ballot could help Republican recruiting efforts as they seek to perform better than their losses in the recent midterm elections. A weak GOP field contributed to Democratic Sen. Mark Kelly’s reelection in Arizona last month.

A political action committee, Primary Sinema, that is raising money to support a potential challenger, said the money it has already raised will now be used to back “a real Democrat” in 2024.

Abandoning the Democratic Party is a striking evolution for a politician who began her career as a Green Party member and antiwar activist known as a “Prada socialist.” The shift has been particularly vexing for progressive activists who now see her as one of their chief antagonists.

In a video explaining her decision, she said: “Showing up to work with the title of independent is a reflection of who I’ve always been.”

The first-term Sinema wrote Friday in The Arizona Republic that she came into office pledging “I would not demonize people I disagreed with, engage in name-calling, or get distracted by political drama. I promised I would never bend to party pressure.”

She wrote that her approach is “has upset partisans in both parties” but “has delivered lasting results for Arizona.”

Ahead of the 2024 elections, Sinema is likely to be matched against a well-funded primary challenger after angering much of the Democratic base by blocking or watering down progressive priorities such as a minimum wage increase and Biden’s big social spending initiatives.

Sinema’s most prominent potential primary challenger is Rep. Ruben Gallego, who has a long history of feuding with her.

The senator wrote that she was joining “the growing numbers of Arizonans who reject party politics by declaring my independence from the broken partisan system in Washington.”

Sinema bemoaned “the national parties’ rigid partisanship” and said “pressures in both parties pull leaders to the edges – allowing the loudest, most extreme voices to determine their respective parties’ priorities, and expecting the rest of us to fall in line.”

“In catering to the fringes, neither party has demonstrated much tolerance for diversity of thought. Bipartisan compromise is seen as a rarely acceptable last resort, rather than the best way to achieve lasting progress,” she wrote.

Along with West Virginia’s Joe Manchin, she has been one of two moderate Democrats in the 50-50 Senate, and her willingness to buck the rest of her party has at times limited the ambitions of Biden and Schumer.

Sinema is a staunch defender of the filibuster, a Senate rule effectively requiring 60 votes to pass most legislation in the 100-member Senate. Many Democrats, including Biden, say the filibuster leads to gridlock by giving a minority of lawmakers the ability to veto.

Last January, leaders of the Arizona Democratic Party voted to censure Sinema, citing “her failure to do whatever it takes to ensure the health of our democracy″ – namely her refusal to go along with fellow Democrats to alter the Senate rule so they could overcome Republican opposition to a voting rights bill.

While that rebuke was symbolic, it came only a few years since Sinema was heralded for bringing the Arizona Senate seat back into the Democratic fold for the first time in a generation. The move also previewed the persistent opposition that Sinema was likely face within her own party in 2024.

Assoicated Press writer Seung Min Kim contributed to this story.