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Sen. Kyrsten Sinema leaves Democratic Party and registers as an independent

“Becoming an Independent won’t change my work in the Senate; my service to Arizona remains the same," Sinema said in a tweet early Friday morning.

Arizona Sen. Kyrsten Sinema has announced she is leaving the Democratic Party and officially registering as an independent.

"In a natural extension of my service since I was first elected to Congress, I have joined the growing numbers of Arizonans who reject party politics by declaring my independence from the broken partisan system in Washington and formally registering as an Arizona Independent," Sinema, 46, tweeted early Friday morning.

"Over the past four years, I’ve worked proudly with other Senators in both parties and forged consensus on successful laws helping everyday Arizonans build better lives for themselves and their families," the Arizona senator said. "Becoming an Independent won’t change my work in the Senate; my service to Arizona remains the same."

Image: Democrats Continue To Negotiate Domestic Spending Bills
Sen. Kyrsten Sinema leaves the Capitol on Oct. 27, 2021. Drew Angerer / Getty Images file

Sinema's announcement came days after Democrats reached a 51-49 majority in the upper chamber following Sen. Raphael Warnock’s victory in Georgia — widening their narrowest of majorities. The new Senate makeup will help Democrats overcome certain procedural hurdles and could also weaken the negotiating power of centrist Democrats like Joe Manchin of West Virginia and Sinema on major Democratic priorities.

The chamber will still functionally be a 51-49 Senate, which means Democrats will have a clear majority and full control of committees and the party won't have to strike another power-sharing agreement with Republicans, a Democratic leadership aide told NBC News. Sinema's committee assignments will be through the Democratic majority, said a Democratic aide and a spokesperson for Sinema.

The Democratic aide said Sinema notified Senate Majority Leader Chuck Schumer, D-N.Y., about her decision to register as an independent on Thursday.

The Arizona independent will be different than the other independents in the Democratic caucus, Sens. Bernie Sanders of Vermont and Angus King of Maine. A Sinema spokesperson said she has never attended Democratic caucus meetings and won't participate in any now. Both Sanders and King, on the other hand, do caucus with Democrats.

White House press secretary Karine Jean-Pierre said in a statement Friday that Sinema has been a key partner on historic legislation signed by Biden including the American Rescue Plan, Bipartisan Infrastructure Law, Inflation Reduction Act, Gun Safety Act and the Respect for Marriage Act.

"We understand that her decision to register as an independent in Arizona does not change the new Democratic majority control of the Senate, and we have every reason to expect that we will continue to work successfully with her," Jean-Pierre said.

Sinema began her political career as a liberal activist and member of the Green Party. After joining the Democratic Party, she served in the Arizona state Legislature and went on to win a seat in the U.S. House in 2012 representing the Phoenix area. Once in Congress, Sinema went her own way and tacked toward the center, even voting for civil rights icon John Lewis for speaker rather than her party’s liberal leader, Nancy Pelosi. Sinema flipped a GOP-held Senate seat in 2018 and continued to carve out a role as a centrist dealmaker over the past four years, but frequently faced threats of a primary challenge from the left.

During her time with the Democrats, Sinema has repeatedly found herself at odds with the party, including rejecting a filibuster rule change earlier this year that would have helped pass a major voting rights bill, one of Biden's top priorities. Her independent streak has also put her in the middle of negotiations on major bills in the past cycle, including on infrastructure spending legislation, gun restrictions and a major Democratic climate, health care and tax bill dubbed the Inflation Reduction Act.

The Biden administration was informed of Sinema’s decision to leave the Democratic Party “midafternoon” on Thursday, a senior administration official said.

“[We] don’t really think it changes much except her re-election path,” the official said, noting that Sinema is up for re-election in 2024.

A group called the Change for Arizona 2024 PAC said they were continuing a campaign launched in September to primary Sinema in the 2024 election cycle when she's up for re-election.

In an op-ed published by The Arizona Republic newspaper, Sinema expanded on her decision, saying that everyday Americans were being "increasingly left behind by national parties’ rigid partisanship, which has hardened in recent years."

"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," she said, adding: "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. Payback against the opposition party has replaced thoughtful legislating.".

Notably, Sinema did not join the rest of the Arizona Democratic delegation on Air Force One this week as President Joe Biden held his first event in the state as president.

Biden did acknowledge Sinema in his remarks, however, saying: “I want to thank Senator Sinema, who can’t be with us today. She’s in Washington working on another major piece of legislation. A tremendous advocate for the people of Arizona and a leader in so many issues important to this state."