Moments after the Senate voted in an unusually bipartisan way to advance a $1tn infrastructure deal, Kyrsten Sinema hobbled on a broken foot to a press conference to mark the occasion. At the podium, the Arizona Democrat was greeted by Rob Portman, a Republican from Ohio with whom she had led negotiations over how to repair ageing roads and bridges and broadband networks.

“You go first,” Sinema whispered.

“No, you go first,” Portman replied.

“No, no,” Sinema insisted. “You go first. It shows bipartisanship. It’s the right thing to do.”

Portman looked unsure. Sinema nodded firmly: “It is the right thing to do.” Portman stepped forward.

Until this year, Sinema had spent her political career in the minority. Working with Republicans was her only option. Now, in a Senate divided 50-50, she has options – and power.

In her new role, Sinema holds an effective veto over her party’s priorities. She is at the center of every major legislative battle, an enigma to many colleagues in Washington, to many who helped elect her she is an impediment to progress.

With the fate of Joe Biden’s historic proposal to expand the social safety net hanging in the balance, Democrats are racing to solve the riddle: what does Kyrsten Sinema really want?

Since arriving in the Senate, Sinema has emerged as one of her party’s most elusive and contentious figures. She has preached bipartisanship, even when it conflicts with Democratic goals. She has vowed to uphold the filibuster, a rule imposing a 60-vote threshold on most legislation.

Though Sinema can stand out in the starchy chamber with her candy-colored wigs (her temporary solution to salon closures during the pandemic) and bold sartorial choices (she once presided over the chamber in a shirt that read “DANGEROUS CREATURE”), she lately prefers not engage publicly. She rarely sits for interviews with the national press and avoids questions from reporters on Capitol Hill.

Earlier this summer, Sinema made clear that she would not support the $3.5tn price tag for Biden’s social policy and climate change bill. But unlike Joe Manchin, a three-term senator from West Virginia who is one of the most conservative Democrats in Congress, she has not publicly detailed her concerns with the legislation.

“Senator Sinema’s position has been that she doesn’t ‘negotiate publicly’ and I don’t know what that means​,” Senator Bernie Sanders, a progressive from Vermont, said this week, calling it “wrong” for her and Manchin to stand in the way of a bill supported by most of the party and the president.

Sinema’s relative silence was even parodied on the season premiere of Saturday Night Live.

“What do I want from this bill?” Cecily Strong asked, playing Sinema. “I’ll never tell.”

Biden has invited Sinema to the White House on multiple occasions. According to her office, she has shared with the president and the Senate majority leader, Chuck Schumer of New York, “detailed concerns and priorities, including dollar figures”. But no deal has been reached and Democrats are growing exasperated.

“Maybe she knows what she’s doing; maybe she’s got a strategy that she’s just not sharing with us; maybe her blueprint is the way to go,” said Yolanda Bejarano, the Phoenix-based national legislative and political field director for the Communications Workers of America, a major union.

“But it’s getting more and more difficult to explain it away.”

Allies and critics have long considered Sinema one of the savviest politicians in the state, in part because she understands the Arizona electorate better than anyone else.

For years, Sinema has honed a brand of centrism that observers say better aligns with the politics of Arizona, a once Republican stronghold shaped by the conservatism of Barry Goldwater, a senator and nominee for president in 1964. Invoking the late senator John McCain as a hero, Sinema promised to be an “independent voice” and appealed to suburban women, independents and disaffected Republicans. In 2018, Arizona duly sent a Democrat to the Senate for the first time in 30 years.

“Her ideological core is pragmatism,” said Chuck Coughlin, a Phoenix-based political consultant who left the Republican party after Donald Trump became president. “She understands that if she is to succeed in Arizona, she must succeed in this lane.”

But she now faces a growing backlash among the party faithful in her home state, the progressive activists and Democratic voters who knocked on doors in brutal summer heat to help get her elected.

Activists with a grassroots organization, Lucha, disrupted a class Sinema was teaching at Arizona State University and followed her into a bathroom, demanding she support Biden’s social policy bill and pass immigration reform. She was confronted again on the plane as she returned to Washington. An online fundraiser to support a potential primary challenge raised more than $100,000 in a few days.

In a scathing statement, Sinema denounced the bathroom incident “not legitimate protest”.

Lucha defended its actions, accusing the senator of becoming “completely inaccessible”. “We’re sick of the political games, stop playing with our lives,” the groups said.

Sinema’s office denied the accusation that she had become unresponsive. “The senator and her team meet regularly with individuals and groups from across Arizona on a consistent basis – including Lucha,” said spokesman John LaBombard, adding that they had met directly with activists from Lucha “at least a half-dozen times” since 2019.

The state Democratic party has threatened a vote of no confidence. Garrick McFadden, a former vice-chair, called Sinema an “obstructionist” and predicted an exodus from the state party by officials eager to replace her.

“She has betrayed her friends and the promise she made to the Arizona people,” he wrote on Twitter. “She wants to play games, well in 2023 we start playing games with her.”

Though Sinema will not face voters again until 2024, there are early signs she may be vulnerable. A recent poll by OH Predictive Insights, a Phoenix firm, found that just 56% of Arizona Democrats had a favorable view of Sinema. Nearly one in three had an unfavorable view

Saundra Cole, a retired ATT telephone operator, is among them. She phone-banked for Sinema several times, first when she ran for the House and later for the Senate. Cole was drawn to her story of personal hardship, and believed she would be a champion for working families. Now the 72-year-old struggles to see how she could vote for her again.

When Sinema delivered an emphatic thumbs-down to a proposal that would have raised the federal minimum wage to $15 an hour, Cole was shocked.

“It was like a slap in the face,” she said.

Some speculated Sinema was channeling McCain, whose last major legislative act was to block his own party’s attempt to repeal the Affordable Care Act with a dramatic thumbs-down. Cole rejected the comparison.

“She’s not John McCain,” Cole said. “She’s not a maverick. I didn’t agree with him on many things but at least we knew where he stood.”

In her telling, Sinema was born to a middle-class family in Tucson. When her father lost his job, the life they built began to unravel. After Sinema’s parents divorced, her mother and stepfather moved to the Florida Panhandle, where she has said they lived in an abandoned gas station for three years. Sinema describes her years living in poverty as formative, guiding her into a career in politics.

“We got by thanks to help from family, church and, sometimes, even the government,” she said in a video that launched her Senate bid.

After graduating from Brigham Young University in just two years, Sinema returned to Arizona, where she started working as a social worker. She first encountered politics as a Green Party activist working for Ralph Nader. She spent the early 2000s agitating against wars in Afghanistan and Iraq. At the time, she condemned capitalism and likened political donations to “bribery”. She ran twice for local office as an independent – and lost.

In 2004, Sinema changed her affiliation to Democratic and won a seat in the state legislature. Her progressive credentials were unimpeachable.

“People ask all the time whether she has changed her views on issues,” said David Lujan, who served with Sinema in the state capitol. “I honestly don’t know. I think only Kyrsten can answer that. What has changed is her approach.”

In her book, Unite and Conquer: How to Build Coalitions That Win – and Last, Sinema details her metamorphosis from “bomb-thrower” to stubborn bipartisan. She recalls railing against bills in floor speeches and interviews, only to watch them pass with overwhelming support in the Republican-held chamber.

“In short, my first legislative session was a bust,” she wrote, calling herself the “patron saint of lost causes”. She returned with a plan, to befriend colleagues, search from common ground and hopefully “be at least marginally more successful”.

In 2006, she helped lead the opposition to a ballot initiative amending the state’s constitution to ban same-sex marriage. Lujan said she applied lessons learned in the legislature to the effort, centering the campaign on how the amendment would impact unwed, elderly straight couples. The strategy was not without its critics, notably from the LGBTQ+ community, in which she identified as a bisexual woman. But the proposal failed, making Arizona became the first state to defeat a same-sex marriage ban at the polls.

“Everybody was shocked that they were able to do it,” Lujan said. The experience, he believes, helped validate her philosophy that “taking a different approach and forming different types of coalitions” could also deliver results.

In 2012, Sinema ran for Congress in a newly drawn district encompassing parts of East Valley, in Phoenix. Republicans tried to use her activist past against her but the strategy failed. Sinema won narrowly, becoming the first openly bisexual member of Congress. She is now the first female senator from Arizona and the only lawmaker on Capitol Hill to claim no religious affiliation – she was sworn in on a copy of the US constitution.

Friends and colleagues in both parties invariably describe her as disciplined and extremely intelligent. While in office, she earned a law degree and an MBA. Away from Washington, she runs Ironman triathlons and teaches college courses on social work.

As Sinema climbed the ranks of Arizona politics, the state was changing. After Barack Obama’s election as president in 2008, anti-immigrant fervor sent Arizona lurching further right. Hope of turning the state blue faded as national Democrats and progressive groups effectively abandoned the state, recalled Josh Ulibarri, a Phoenix-based Democratic pollster.

“Everybody left,” he said.

In the vacuum, a liberal grassroots resistance emerged, led by young Latinos who were the targets of harsh immigration policies. They slowly built political clout. Their advocacy helped oust Sheriff Joe Arpaio in 2016, and to pass the largest minimum-wage increase in the country. Two years later, they helped send Sinema to the Senate.

“She benefited from their work, their struggle – the sweat and the blood, the arrests and the deportations – all of it,” Ulibarri said. “So the way she’s become the public face of [resistance to Biden’s agenda], it’s not just a political betrayal, it’s a deep, personal betrayal.”

Frustration reached a turning point this summer after Sinema doubled down in her support for the filibuster, which Republicans have used to block voting rights legislation. Two civil rights leaders, the Rev Jesse Jackson and the Rev William Barber, were arrested during a protest outside of her Phoenix office.

Gilbert Romero, a prominent progressive activist in Phoenix who interned for Sinema in 2014, said he doesn’t see such anger abating anytime soon, especially if she continues to stand in the way of Democratic policy goals. In his view, Sinema underestimates the threat of a progressive primary challenge.

“She thinks she’s like Teflon and nothing is going to stick to her – that’s misguided,” he said, adding: “We’ve [unseated] much more powerful people than Kyrsten Sinema.”

What many activists find baffling is that Sinema has moved right as her state has moved left. In 2020, Biden narrowly won Arizona, as did Mark Kelly, who now serves alongside Sinema in the Senate. Democrats also hold five of the nine congressional seats. Manchin, the other key holdout, is the only Democrat left in the congressional delegation from West Virginia, a state that voted for Trump by nearly 40 points.

Confounding still, they say, is that the party’s massive social policy package is popular, despite ​unified Republican opposition​ in Congress​.

A survey of Arizona voters by the progressive polling firm​,​ Data for Progress​,​ found broad support for the measure,​ ​which includes provisions to make child care and healthcare more affordable as well as programs to combat climate change — a major concern ​for residents of one of the nation’s hottest and driest states. Arizonans also strongly approved of plans to pay for the bill by raising taxes on corporations and wealthy Americans, parts of which Sinema has reportedly objected to.

“The Arizona that existed in 2012 when she first got elected to Congress is not the Arizona that exists in 2021,” Romero said. “It’s a completely different landscape now.”

Independence is a prized quality in Arizona, where nearly a third of the electorate is unaffiliated with either major party. Sinema has it, said Danny Seiden, president and chief executive of the Arizona chamber of commerce, who believes it is the source of her broad appeal.

“It’s a willingness to listen and not just toe the party line on all issues,” he said. “I think that’s a rarity amongst both Democrats and Republicans these days.”

His organization supports the bipartisan infrastructure deal but has concerns about the size of the spending package. Seiden had no guesses as to how Sinema will eventually vote, but expects she will “do what she thinks is right for Arizona”.

How she chooses to proceed will almost certainly have long-reaching consequences for her party’s legislative ambitions, and possibly her own. It may also decide the fate of the infrastructure deal she negotiated, which risks becoming collateral damage if talks on the social policy package fail.

According to her office, Sinema is engaged in “good-faith discussions with both President Biden and Senator Schumer to find common ground”, as Democrats work to trim the package and win her support.

Finally in the majority, and holding the key to her party’s ambitious agenda, Sinema has an opportunity that she could​ only have dreamed of​ when she first ran for office, said David Lujan, her former colleague in the state legislature. ​He doesn’t believe she would squander it.

“It would counter my entire understanding of why she changed her approach in the first place,” he said. “She wants to be seen as somebody who can get things done.”