Upon learning that the Arizona senator Kyrsten Sinema was leaving their party, some Democrats’ reactions could best be summed up with two words: good riddance.

The lawmaker has been a thorn in their side since the early days of Joe Biden’s presidency, snarling negotiations over the White House’s priorities and voting down reforms dear to progressives such as raising the minimum wage to $15 an hour, and reforming the Senate filibuster.

“Senator Sinema may now be registered as an Independent, but she has shown she answers to corporations and billionaires, not Arizonans. Senator Sinema’s party registration means nothing if she continues to not listen to her constituents,” the state’s Democratic party chair, Raquel Terán, said in a statement.

Yet, much like Joe Manchin, the centrist West Virginia senator who has played a similar spoiler role over the past two years, Sinema has come through for her fellow Democrats in many key areas, supporting Biden’s policy positions 93% of the time, according to the political aggregator FiveThirtyEight.

Whether she will continue to offer that help for the next two years is up in the air, after announcing on Friday she had left the party and registered as an independent, a decision that brought to the surface many Democrats’ bitterness towards the first-term lawmaker.

The announcement rocked the party, which had been on something of a roll over the past weeks. On Tuesday, they succeeded in getting every single one of their senators re-elected for the first time since 1934 after Raphael Warnock won his seat in Georgia, and last month only narrowly lost the House of Representatives in the midterm elections.

After spending two years navigating a fraught 50-50 Senate split in which the vice-president, Kamala Harris, came in to break tie votes, they won a new seat in Pennsylvania, and were planning to take outright control of Congress’s upper chamber.

Sinema was vague about the degree to which she would continue to cooperate with the Democrats. In an interview with Politico, the senator said she would not join the Republican caucus, and indicated she would continue voting as she has since first joining the Senate in 2019. “Nothing will change about my values or my behavior,” she told the publication.

Those signals probably explain the far more muted reaction of the Democratic party’s key power players – no doubt now keen to keep her onside as much as possible.

The chamber’s Democratic leader, Chuck Schumer, downplayed the disruption her exit caused, saying in a statement that Sinema “asked me to keep her committee assignments and I agreed. Kyrsten is independent; that’s how she’s always been. I believe she’s a good and effective senator and am looking forward to a productive session in the new Democratic majority Senate.”

The White House stuck a similarly constructive note, perhaps hoping they could continue working with her, as they do with Bernie Sanders and Angus King, two independent senators who vote with Democrats.

“Senator Sinema has been a key partner on some of the historic legislation President Biden has championed over the last 20 months, from the American Rescue Plan to the Bipartisan Infrastructure Law, from the Inflation Reduction Act to the Chips and Science Act, from the Pact Act to the Gun Safety Act to the Respect for Marriage Act, and more,” the press secretary, Karine Jean-Pierre, said.

“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.”

Sinema did indeed support those pieces of legislation, but she has become best-known for what she has not supported: changing the Senate’s filibuster rules to ensure voting rights legislation can pass, various aspects of Biden’s failed Build Back Better Act, and raising the federal minimum wage to $15 an hour, to which she expressed her rejection with an eye-catching thumbs-down delivered on the Senate floor.

She was sometimes joined in these stands by Manchin, who has made little secret of his support for the fossil fuel industry relative to other Democrats, or hesitancy to break Senate tradition.

Sinema’s motives were always harder to discern. She started her political career with the environmentalist Green party, but when the agreement to pass the climate change-fighting Inflation Reduction Act was announced over the summer, she withheld her support until a provision raising taxes on profits that go to the heads of private equity firms was removed.

Voters appear to have noticed. A September poll from AARP Arizona showed her approval underwater with every single group of voters – including Democrats, with whom her favorability rating was a dire 37%. She was likely to face a challenge in the 2024 Democratic primary, probably from the Democratic House representative Ruben Gallego.

“At a time when our nation needs leadership most, Arizona deserves a voice that won’t back down in the face of struggle. Unfortunately, Senator Sinema is once again putting her own interests ahead of getting things done for Arizonans,” Gallego said in a statement released after the senator’s departure.

Arizona’s Democrats are now in a tough position. They will have to decide whether to run a candidate against Sinema in two years, and risk splitting their voters in a state where the GOP is seen as having a good shot at retaking her seat. Blunting a primary challenge may have been Sinema’s ultimate goal in leaving, but it’s hard to know. Even her supporters seem to have trouble understanding how to advocate for her.

“A friend is a big Sinema supporter, someone she knows by name,” Tony Cani, the deputy director of Biden’s Arizona campaign tweeted. “They ran into each other at an event and after warm greetings this person asked: ‘I wanna be helpful, what can I tell my people when they complain about what’s going on?’ Sinema smiled and said: ‘Tell them to fuck off.’”