Arizona Sen. Kyrsten Sinema on Friday revealed she’s changed her party affiliation from Democrat to independent, a blow for Democrats celebrating their recently secured clear Senate majority.

In an interview with Politico, Sinema indicated that she would continue to vote the same way she has over her previous four years in the Senate and said that she did not intend to caucus with Republicans. “Nothing will change about my values or my behavior,” she said.

But the news will surely disappoint Democrats looking forward to a clear 51-seat Senate majority in the next Congress. President Biden’s party will still hold a workable majority, where Vice President Kamala Harris will retain a tie-breaking vote in 50-50 splits, and Democrats will still be expected to have the necessary votes to control Senate committees.

Sinema would not confirm if she plans to run for re-election in 2024 and said she had informed Senate Majority Leader Chuck Schumer about her affiliation switch on Thursday.

“I don’t anticipate that anything will change about the Senate structure,” Sinema said, adding, “I intend to show up to work, do the same work that I always do. I just intend to show up to work as an independent.”

Speaking about her decision to leave the Democratic Party, Sinema explained that she has “never really fit into a box of any political party” and that becoming an independent represented the next logical step in a career characterized by working closely with both sides of the aisle.

Sinema’s Democratic colleagues have previously criticized her reluctance to toe the party line on issues like raising tax rates and weakening the filibuster, but her bipartisanship has also allowed her to play a key role in getting agreements on subjects like gun control and same-sex marriage over the line. She was notably absent last month during Sen. Mark Kelly’s (D-AZ) successful re-election campaign.

Her defection is the first Senate party switch since 2009, when Pennsylvania Sen. Arlen Specter changed his affiliation from Republican to Democrat. Sen. Joe Manchin (D-WV) regularly shuts down speculation that he too is planning to leave the Democratic Party.

Sinema’s defection, however, means that he will continue to hold some of his crucial voting power, which he has repeatedly wielded to thwart major parts of Biden’s policy agenda in recent years. Both Manchin and Sinema pushed back against the multitrillion-dollar Build Back Better bill proposed by Biden last year and have been considered an obstacle by fellow Democrats. In January, Rep. Alexandria Ocasio-Cortez (D-NY) slammed Sinema as “a profound ally” to corporate interests after Sinema blocked voting rights legislation passing through Congress.

News of Sinema’s switch comes just days after Democrats celebrated Sen. Raphael Warnock’s midterm election win in Georgia as the 51st seat needed in order not to have to rely on the vice president’s tiebreaking vote. Now control of the Senate is somewhat more precarious than Dems would have hoped.

“Partisan control is a question for the partisans,” Sinema said, “and not really one for me.”