Arizona Sen. Kyrsten Sinema leaves Democratic Party to become independent

Arizona Sen. Kyrsten Sinema has switched parties to become an independent, complicating the Democrats' narrow control of the U.S. Senate.

Sinema said in a tweet Friday that she was declaring her "independence from the broken partisan system in Washington and formally registering as an Arizona Independent."

A senior Biden administration official told NBC News that the White House learned of Sinema's intention to switch parties "mid-afternoon Thursday." The official said the White House was told Sinema still intends to caucus with Democrats as an independent, like Senators Bernie Sanders of Vermont and Angus King of Maine do.

If Sinema still caucuses with Democrats, her switch to independent would not change much about how the party functions with a 51-49 majority. She is expected to keep her committee assignments through affiliation with Democrats, according to a Democratic aide.

Sinema told Senate Majority Leader Chuck Schumer, D-N.Y., about her plans to become independent on Thursday.

Sinema and Sen. Joe Manchin of West Virginia have been wild cards for Democrats since the party gained narrow control of the Senate from Republicans in 2020. Both had an outsize role in policymaking, as Manchin significantly curbed Democrats' dreams of passing sweeping legislation. Neither senator was up for reelection until 2024 and many expect Manchin to lean further conservative now that the midterms have passed.

Sinema had exerted her own influence on major Democratic bills even before she left the party. She notably rejected a corporate tax increase as part of Democrats' Inflation Reduction Act passed earlier this year, instead opting for a 15% minimum tax.

Sen. Raphael Warnock's reelection win Tuesday in Georgia's U.S. Senate runoff election gave the Democrats one more vote in the chamber and boosted the party's hopes that a 51-49 majority in the Senate would give Sinema and Manchin less control on crucial bills. The chamber was previously split 50-50, with Vice President Kamala Harris casting the tiebreaking vote.

Sinema, who shared her party switch with a handful of news outlets along with her tweets at 6:01 a.m. ET, prides herself on "maverick" behavior like her Arizona predecessor, the late Sen. John McCain. She has made a career in the chamber by trying to work with Republicans as frequently as she did her former party, and told Politico in an interview Friday that switching party affiliations was a logical next step for her.

"Registering as an independent is what I believe is right for my state," Sinema said in the interview. "It's right for me. I think it's right for the country."

Sinema, a 46-year-old and the first openly bisexual senator, was not always the conservative-leaning Democrat that her last four years legislating would indicate. She has always maintained an independent streak and continues to buck Senate norms with colorful outfits and wigs.

Sinema started her career as a Green Party activist focusing on LGBTQ rights. She switched to the Democratic Party in 2004 and was elected to the U.S. House in 2012.

Sinema utilized her friendliness with Republicans to be a key broker on several signature bills of President Joe Biden's first term, aiding on issues including infrastructure, guns and same-sex marriage. But her views on increasing taxes on the wealthy and opposition to changing filibuster rules did not win her favors with her former party.

She notably rejected a corporate tax increase as part of Democrats' Inflation Reduction Act passed earlier this year, instead opting for a 15% minimum tax.

Long before her announcement Friday morning, some Arizona Democrats had already started trying to find a replacement to primary her. Groups like the Primary Sinema PAC emerged late last year after her reluctance to filibuster reform prevented Democrats from moving forward with an exception for voting rights legislation, leading to the central committee of the Arizona Democratic Party to issue a no-confidence vote in its senator.

Primary Sinema PAC does not support a single candidate, but rather funds local Arizona groups to pressure Sinema and to lay the groundwork for the candidate that emerges. Speculation had already started that Rep. Ruben Gallego, D-Ariz., would challenge her.

Sinema's decision to switch parties would prevent her from having to face a primary from the left.

In her interview with Politico though, Sinema did not say whether she would seek a second term in the U.S. Senate: "It's fair to say that I'm not talking about it right now."