Kyrsten Sinema Says She Will Leave the Democratic Party

WASHINGTON — Senator Kyrsten Sinema of Arizona announced on Friday that she would leave the Democratic Party and become an independent, unsettling the party divide anew just days after Democrats secured an expanded majority in the Senate.

“I have joined the growing numbers of Arizonans who reject party politics by declaring my independence from the broken partisan system in Washington,” she wrote in an op-ed published in The Arizona Republic.

Ms. Sinema, who was already facing a likely Democratic re-election challenge in 2024 after angering her party by opposing key elements of its agenda, will need to maintain some ties with Democrats to keep her committee seats and other benefits, since they would still hold a majority without her.

She informed Senator Chuck Schumer, the New York Democrat and majority leader, of her plans on Thursday, according to a Senate Democratic aide who described the private conversation on the condition of anonymity. The aide said that Ms. Sinema would keep her committee positions through Democrats, meaning the party would still hold a one-seat edge on the panels next year, giving them new flexibility over nominations and legislation.

Still, her decision will no doubt ruffle Democrats who on Wednesday were exuberant about securing a 51st seat in the Senate with Senator Raphael Warnock’s victory in Georgia. Ms. Sinema’s switch is likely to provide new complications for Mr. Schumer and Democrats going forward, even though she wrote in her op-ed that “becoming an independent won’t change my work in the Senate; my service to Arizona remains the same.”

Ms. Sinema has not said whether she would caucus with the Democrats, as do two other independent senators, Bernie Sanders of Vermont and Angus King of Maine. She told Politico that she would not caucus with Republicans, and that her ideology and voting habits would not change.

Party switching is not unheard-of in the Senate when lawmakers see political advantage in making such a move. After losing in a Democratic primary in 2006, Senator Joseph I. Lieberman of Connecticut ran and won as an independent but continued to caucus with majority Democrats. Senator Arlen Specter of Pennsylvania left the Republican Party in 2009, after joining with Democrats in pushing some initiatives of the Obama administration, but was later defeated in a Democratic primary.

Ms. Sinema has cast herself as a bipartisan deal-maker in the Senate and is often seen on the Republican side of the floor, conversing with and lobbying Republicans with whom she has worked on a variety of issues. Like Senator Joe Manchin III, Democrat of West Virginia, she has been a holdout on some major Democratic priorities such as tax increases. She and Mr. Manchin killed Democratic efforts to weaken the filibuster and push through new voting rights legislation this year. Arizona Democrats symbolically censured her after her filibuster vote.

Writing in The Arizona Republic, Ms. Sinema said that she had “never fit perfectly in either national party” and that the “loudest, most extreme voices continue to drive each party toward the fringes.”

“When politicians are more focused on denying the opposition party a victory than they are on improving Americans’ lives, the people who lose are everyday Americans,” she wrote. “That’s why I have joined the growing numbers of Arizonans who reject party politics by declaring my independence from the broken partisan system in Washington.”

This is a developing story. Check back for updates.