Sen. Kyrsten Sinema of Arizona said in remarks published at 6 am ET Friday that she's leaving the Democratic Party and registering as an independent.

Why it matters: It's a political earthquake that will shake up the Senate — and a gut punch for Democrats just three days after they secured a 51-49 majority.

Our thought bubble, via Axios' Josh Kraushaar: Sinema views activists in the Arizona Democratic Party as extreme as the state Republican Party.

• She's up in 2024, and risked a primary from Rep. Ruben Gallego (D-Ariz.) on the left.

"Arizonans — including many registered as Democrats or Republicans — are eager for leaders who focus on common-sense solutions rather than party doctrine," Sinema says in an op-ed in The Arizona Republic, her state's largest paper.

• "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."

Sinema — ever unpredictable and inscrutable — told Senate Majority Leader Schumer of her decision yesterday, a Democratic aide told Axios' Andrew Solender.

• Schumer said Sinema will keep her committee assignments in a statement later Friday, adding, "Kyrsten is independent; that’s how she’s always been."
• "We will maintain our new majority on committees, exercise our subpoena power, and be able to clear nominees without discharge votes," the New York Democrat said.

"I know some people might be a little bit surprised by this," Sinema told CNN's Jake Tapper. "But, actually, I think it makes a lot of sense. ... I've never fit neatly into any party box. I've never really tried. I don't want to."

• "Removing myself from the partisan structure — not only is it true to who I am and how I operate, I also think it'll provide a place of belonging for many folks across the state and the country, who also are tired of the partisanship."

Reality check: Punchbowl News notes that Sinema leaving the party gives Sen. Joe Manchin (D-W.Va.) "outsized sway once again."

What they're saying: "Senator Sinema has been a key partner on some of the historic legislation President Biden has championed over the last 20 months," White House press secretary Karine Jean-Pierre said in a statement.

• "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."

Worth noting: Gallego teased a Senate bid Friday after Sinema's announcement, saying he was “thinking of running," per a fundraising text his campaign sent out.

Go deeper: Read Sinema's op-ed

Editor's note: This article has been updated with new details throughout.