Sinema’s switch was months in making. Now it poses a challenge for Democrats.

The unhappiness has run in both directions and loomed over Sinema’s political future, prompting some critics to see a calculated ploy for survival in Sinema’s announcement. She is unpopular with Democrats back home following some high-profile party-line defections. Polls also suggested she could lose a Democratic primary if she sought reelection in 2024 — a hurdle she would no longer need to clear as an independent.

Now Sinema, 46, has caused what many see as the first big potential political earthquake in the battle for the Senate in 2024. Her shift is the latest of several reinventions throughout her career, as she has climbed the ladder from Green Party activist to state and eventually federal lawmaker with far less liberal positions. Although she has not said whether she will run for a second term, Sinema’s most recent conversion carries significant ramifications in a key battleground.

“There are certainly some who have wanted me to fit into one box or the other,” Sinema said in an interview. “But I have never wanted to do that.”

Sinema had been weighing a departure from the Democratic Party for months, people familiar with the timing said. By the fall, planning was underway, even before she appeared in late September at the University of Louisville McConnell Center, where she defended her centrist brand of politics — and further enraged some liberals — with the Republican Senate minority leader, Mitch McConnell of Kentucky, looking on.

McConnell advises Sinema frequently, according to a person familiar with their relationship, and the two spoke about her decision to defend the legislative filibuster and other matters. “She and I talk all the time,” McConnell told reporters Tuesday.

High-ranking Democrats said they remained in the dark until just before she pulled the trigger. Senate Majority Leader Charles E. Schumer (D-N.Y.) learned she would be leaving the party the day before she announced it, he said recently. White House aides have declined to say whether President Biden received a heads up, or how he reacted when he was told she would not be joining him on Air Force One for his visit to Arizona a few days before she broke the news. (Biden said at the time that Sinema needed to stay in Washington to work on legislation, calling her a “tremendous advocate” for Arizona.)

Sen. Mark Kelly (D-Ariz.) declined to say whether Sinema warned him that she was leaving the party and, like most Democrats, did not answer a question about whether he would support her if she runs for reelection as an independent. “You are getting into hypotheticals,” he said last Monday. “But I have worked very closely with her for a long period of time.”

A marathon runner and triathlete who has broken the mold of the typical senator, Sinema often eschewed Democratic events and meetings, frequently setting up her own bipartisan negotiations with Republicans and centrists on legislation. The first openly bisexual person to serve in the Senate, her latest negotiation resulted in federal legislation to protect same-sex couples that attracted 12 Republican votes in the Senate.

“She’s always been an independent thinker,” said Sen. Chris Murphy (D-Conn). “This seems like this changes the letter next to her name and not much else.”

Where some have seen a maverick streak in line with Arizona’s history of rewarding political independence, others have seen a betrayal in her disconnect from local party officials and activists who supported her election as a Democrat as she sought her Senate seat.

“There has been zero engagement since she got elected,” Stephen Slugocki, a former chair of the Maricopa County Democratic Party, said of the senator and her team. “No relationship. They haven’t been involved in anything — they’ve lost all communication. People feel disappointed that they worked hard to elect her and this is what they’ve got in return.”

For her part, Sinema has said that her decision is not political and that she has not yet made a decision about whether she will seek reelection, even as she has filed paperwork to run. She declined to pinpoint a specific moment when she decided to change her party ID.

“I’ve always been independent,” Sinema said. “I can’t point you to any specific instance because there isn’t one.”

‘She told us who she was going to be’

A former social worker, Sinema entered politics not as a Democrat, but as a member of the Green Party, where she worked as a spokesperson and organized antiwar protests.

Records obtained by The Washington Post show that after losing a bid for city council, she registered as an independent voter, joining the growing ranks of Arizonans without a political home. She then pursued a seat in the state legislature as an independent affiliated with the Green Party, but lost.

By 2004, she had registered as a Democrat, won a seat in the state House and displayed a liberal streak in a legislature dominated by Republicans. Over time, she moved to the center and forged relationships across the spectrum.

“My first legislative session was a bust,” she wrote in her 2009 book, “Unite and Conquer: How to Build Coalitions That Win and Last.”

She continued, “I’d spent all my time being a crusader for justice, a patron saint for lost causes, and I’d missed out on the opportunity to form meaningful relationships.”

Republicans, she wrote, were a “discipline machine” on messaging their priorities while liberals were too “free-flowing,” to their detriment. “This is not home decorating — you do not need a flourish,” she wrote, adding, “winning is the best flourish of all.”

She adhered to that approach after clinching a toss-up seat in the U.S. House in 2012, where she was known as one of the chamber’s most moderate Democrats, and again during a bitter campaign in 2018 for an open U.S. Senate seat. Sinema rarely described herself as a Democrat. Instead, she told voters, her focus would be to “get stuff done.”

But her first few years in Washington were a disappointment to many Democrats, even though she rarely strayed from the party line. She voted to confirm William P. Barr as attorney general and David Bernhardt as interior secretary during Donald Trump’s presidency, earning her condemnation from party activists.

In an evenly-divided Senate, Sinema, along with Sen. Joe Manchin III (D-W.Va), torpedoed Democrats’ $3.5 trillion climate and social spending “Build Back Better” plan, objecting to its price tag. (She eventually supported a $1.7 trillion version, after demanding changes to a proposed tax for private equity executives that angered some liberals.)

“I don’t think she went there thinking she was going to be a traditional Washington Democrat,” said Kirk Adams, a Republican and Sinema friend and former chief of staff to Gov. Doug Ducey (R). “She told us who she was going to be.”

Sinema has largely shunned the Arizona Democratic Party and its apparatus. She routinely skipped party conventions and fundraising dinners, an approach some liberal activists hoped would change after they helped elect her to the Senate.

By early 2021, anger with Sinema among Democrats boiled over after her vote to keep an increase in the federal minimum wage out of a pandemic relief package — which she cast with a conspicuous thumbs down motion — and her continued opposition to eliminating the Senate’s legislative filibuster, which requires 60 votes for most legislation to pass.

She posted a photo of herself on social media last year drinking sangria and wearing a ring she bought from a local boutique that read “F— off.”

Fury at Sinema was reflected at the very top of the state party. Brianna Westbrook, an official with the party, tweeted that Sinema was a “villain that is opposed to democracy” last year when she opposed getting rid of the filibuster, and encouraged her staff to quit their jobs. Other party officials amplified criticism of her online.

Angry protesters confronted her at Arizona State University last year over her Build Back Better objections, following her into a bathroom while recording her on a phone. She was also confronted on a plane and at airports. Previously unguarded, she began traveling with security guards. Campaign finance records show her…