PHOENIX — Kyrsten Sinema won Democrats a U.S. Senate seat from Arizona for the first time in a generation thanks in no small part to unity in her party and division among Republicans.

That Democratic unity of 2018 was on display again in the next two election cycles as the party picked up Arizona’s other Senate seat and won the top three state offices.

But that winning formula is in jeopardy ahead of the 2024 election because of Sinema’s estrangement and subsequent divorce from the Democratic Party, which could complicate President Joe Biden’s path to reelection and the party’s hopes for maintaining control of the Senate. She registered as an independent shortly after last year’s midterm elections.

Democrats are already voicing fears that a three-way race with Sinema picking up votes from both Democrats and independents could hand the seat to a Republican such as Kari Lake, the failed gubernatorial candidate and one of the country’s most prominent election deniers.

“If there were ever a time for her to listen to her constituents for once, it’d be now,” said Alex Gomez, executive director of the Latino organizing group Living United for Change in Arizona, which has tangled with Sinema for years. “She needs to step aside. The potential candidacy of a Kari Lake presents a clear and present danger to our democracy.”

Sinema has not said whether she will seek reelection, and Lake has not announced a Senate campaign. But the race already has a Democratic candidate in U.S. Rep. Ruben Gallego, a Latino military veteran who kicked off his campaign last month after spending years as one of Sinema’s chief antagonists.

Gallego says he raised more than $1 million on his first day in the race, capitalizing on pent-up anger with Sinema among Democrats.

The Senate race is not the only new sign of Democratic division in the state. The Arizona Democratic Party last month had its first contested election for chair in 12 years, pitting a candidate backed by Gov. Katie Hobbs against one backed by most of the state’s other elected Democrats.

The party elected longtime union leader Yolanda Bejarano, who was endorsed by U.S. Sen. Mark Kelly, Gallego and others, bucking the tradition of deferring to the preference of a Democratic governor. Hobbs said Thursday she had not yet spoken to Bejarano — nearly a week after the election.

The party discord in Arizona reverberates beyond the state.

Next year, Democrats, who have a narrow 51-49 Senate majority, are defending seats in 23 states — including seven where Donald Trump won at least once. That includes Arizona, where Trump won in 2016 but where Biden became the first Democratic presidential candidate to carry the state in more than two decades.

Sinema’s political career began with roots in the progressive left and antiwar movement. She first ran for office as a Green Party candidate and lost badly, later winning a state legislative seat as a Democrat. She remade herself as a moderate in the U.S. House and parlayed that reputation into a Senate victory.

Her 2018 Senate win was fueled by a number of factors, including the state’s changing demographics, contempt for Trump among suburban women and Sinema’s spending advantage over Republican Martha McSally.

But McSally’s 2018 campaign strategists laid some of the blame for her loss on Democratic unity behind Sinema and Republican infighting. With Democrats in lockstep, Sinema had a head start on reaching out to swing voters, while McSally focused on holding the GOP together to win her primary, campaign officials wrote in a memo that circulated widely after the election.

When Sinema was sworn into office in 2019, Trump was in the White House, Republicans were in control of both chambers of Congress and Democrats were unified in opposition.

But her relationship with the party ruptured during Biden’s presidency as she teamed up with fellow moderate Democratic Sen. Joe Manchin of West Virginia and became a roadblock for parts of the president’s agenda and many progressive priorities.

She is one of the Senate’s most vociferous defenders of the filibuster rule, which requires 60 of 100 votes to pass most legislation, and which many Democrats say empowers Republicans to overrule the will of the Democratic majority.

Sinema says she’s focused on crafting bipartisan deals that can outlive any one party’s control of Congress and points to victories, including a massive infrastructure bill and protections for same-sex marriage.

Her transformation from liberal rabble-rouser into Democratic irritant has left the base feeling angry and betrayed just four years after her victory brought Arizona Democrats in from the cold.

“As long as Sinema’s off the team, that’s all that matters,” said Dave Crose, a 67-year-old retired mechanical engineer from Sun City who voted for Sinema in 2018 but has grown disillusioned with her.

“That’s a bad thing to say, but she screwed everyone in the state, so payback’s her hell,” he said.

For Democrats, long shut out from the halls of power, winning was enough to paper over ideological divides, but now they have shown they can win and it wasn’t a fluke.

“When you have power, everyone wants a piece and there’s actually something to fight over,” said Barrett Marson, a Republican political consultant in Phoenix aligned with the party’s establishment wing.

One thing Democrats have going for them: Republicans aren’t likely to be any less divided than they’ve been since Trump took over the party in 2016.

With the Sinema vs. Gallego drama raising some uncomfortable questions, Democrats in Arizona and Washington have tried to put off picking sides.

Senate Majority Leader Chuck Schumer, D-N.Y., said after Gallego’s announcement that “it’s much too early to make a decision” about the 2024 race.

Kelly also declined to wade too deeply into the dynamics of a potential three-way race, saying “there’s plenty of time” to sort it out.

“I’m not going to get ahead of Sen. Sinema on this,” he told reporters at the Capitol. “I’m going to work with both of of them.”

Notably, however, Hobbs subtly suggested she would not be supporting her old friend Sinema. Hobbs and Sinema are both former social workers who campaigned together for the state Legislature a decade ago, Sinema getting elected to the Senate and Hobbs to the House.

Congratulating the new Arizona Democratic Party leadership team on Twitter, Hobbs wrote that she looks forward to helping the party “win back our US House & Senate seats.”