(CNN) Sen. Kyrsten Sinema is at the zenith of her influence in Washington, with the ability to dramatically reshape -- or derail -- President Joe Biden's domestic agenda simply by threatening to point her thumb down.
But how she has handled her growing power in the 50-50 Senate has also cost her something else: support from the left in Arizona, with progressives warning she is now at risk of becoming a political pariah and is potentially vulnerable in a 2024 Democratic primary battle.
"I think the sentiment that I'm hearing out there, voters in Arizona are upset with her, especially Democratic voters," Rep. Ruben Gallego, a fellow Arizona Democrat, told CNN on Wednesday. "I think they support the President's agenda, and they hope that she will, in the end, support the President's agenda, pass reconciliation -- the Build Back Better agenda."
Gallego, who has been frequently mentioned as a potential primary challenger against her in 2024, pointedly did not rule out a run against Sinema.
"For me, all I care about is what happens between now and 2022," Gallego said. "Those questions wait, wait."
The criticism on the left is rampant not only back home but also in the halls of the US Capitol, as Sinema has stood firm against Democratic calls to gut the Senate's filibuster to pass voting rights legislation and joined other moderates in voting against a minimum wage hike earlier this year.
Sinema is not the only Democrat who has infuriated the left. West Virginia Sen. Joe Manchin has refused to go along with many Democratic priorities -- such as generous paid family and medical leave policies, aggressive targets to reduce greenhouse gas emissions --- opening him to constant criticism from liberals in Congress and activists across the country.
Yet Sinema's handling of the situation is markedly different. For months, she has operated in near secrecy, privately holding conversations with the White House about her concerns. She has provided little information to fellow Democrats, other than Senate Majority Leader Chuck Schumer, whereas Manchin has engaged directly with lawmakers and even spent Tuesday at a caucus lunch fielding a barrage of questions and hearing concerns from his Democratic colleagues. Sinema skipped the lunch, meeting with White House officials instead.
"There is a sense in which we no longer live in a democracy; we live under the tyranny of Kyrsten Sinema," said Rep. Ritchie Torres, a progressive New York freshman. "I welcome the ideological diversity of the party. I can live with dissent. My colleagues and I have trouble living with what we perceive to be erraticism. The perception of erraticism is brought on by a lack of communication and clarity for where she stands."
A spokesman for Sinema declined to comment. But aides have noted that she has been clear about her positions with Schumer and Biden since August.
"While we do not negotiate through the press -- because Sen. Sinema respects the integrity of those direct negotiations -- she continues to engage directly in good-faith discussions with both President Biden and Sen. Schumer to find common ground," spokesman John LaBombard said in a statement last month.
Over the past 20 years, the 45-year-old Sinema has transformed from a Green Party, anti-war activist who first ran for office as an independent to a moderate Democratic senator bent on bipartisanship. She now views the late GOP Sen. John McCain, who sparred with his party and reached across the aisle during high-profile debates, as a role model.
As Sinema has emerged as a power player in the divided Senate, she took a McCain-like lead role in cutting a deal on a sweeping infrastructure bill to pump new money into roads, bridges and broadband. The roughly $1 trillion plan won the support of 19 Senate Republicans.
"I don't think we would have gotten that across the finish line without all of her hard work," Sen. Mark Kelly, a freshman Arizona Democrat, said Wednesday. "I think without her it wouldn't have happened, so she deserves a ton of credit for that."
But Sinema has revealed even less publicly during the frenzied debate over the massive social safety net bill, often walking in silence to the nearest elevators when she faces reporters' questions in the Capitol, with her aides saying that she won't negotiate in public. It's that silence that has irked many of her previous allies -- and it even provoked a viral incident where activists followed her into a women's bathroom at Arizona State University.
The entire Democratic agenda now hinges on Sinema since any one defection in the Senate would be enough to torpedo the President's so-called Build Back Better plan. She, along with Manchin, balked at the plan's $3.5 trillion price tag -- the main reason why it was slashed to now near $2 trillion.
According to Democratic sources, Sinema continues to resist higher taxes on corporations and individuals that would raise hundreds of billions of dollars in revenue, raising major questions on how the bill will be fully paid for -- as the White House has repeatedly promised.
"Her position is well known," Speaker Nancy Pelosi said Thursday when asked by CNN if the Arizona senator has conveyed concerns about tax hikes directly to her.
Washington Rep. Pramila Jayapal, a progressive leader in Congress, is at a loss over why Sinema would be opposed to those higher taxes on the wealthy, which would partially reverse the Trump-era cuts Sinema voted against as a member of the House in 2017.
"This is probably the most popular part of the whole package, is the tax pieces of it, and it's hard to understand," said Jayapal, who met with the senator recently but learned little of her policy stances.
"She's really negotiating with the President and, so I feel like what I have is what the President has said to me, which is, she is very much at the table," Jayapal added.
Democratic sources told CNN that Sinema's resistance to a top Biden priority -- tuition-free community college -- was a chief reason why that will almost certainly be dropped from the final package.
Sinema, along with a handful of other Democrats in both chambers, also opposed a provision to allow Medicare to negotiate lower drug prices, a plan fought by the pharmaceutical industry. And Sinema has continued to demand that the House first approve the Senate's $1.2 trillion infrastructure bill, which she helped negotiate and which liberals are holding hostage until they get an agreement on the larger social safety net plan.
Even some Democratic members of her state's delegation take issue with her approach.
"I think she has a responsibility to everyone and the people in Arizona in particular to tell us what it is she thinks she needs to be done," Rep. Raul Grijalva, a progressive from southern Arizona, told CNN on Wednesday. "And while she doesn't make that declaration, I think it puts her in a more and more difficult situation because there's no white or black smoke here. Tell us what you want, and tell us why you want to cut something."
Asked if he believes she could be vulnerable in a primary, Grijalva said: "I think the consequences of what happens in the next few days, is going to make her or not make her vulnerable."
The Democratic congressman added: "If we don't attack issues of inequity, and we don't really deal with climate change, then I think you start talking about reckonings."
Kelly, however, encouraged caution when asked about the criticism she's received from other Arizona Democrats over the massive economic package.
"This thing isn't over yet," the senator said.
The final phases of the negotiations could be critical to her constituents and powerful organizations back home.
In September, AARP Arizona requested a meeting with Sinema after learning that she didn't support allowing Medicare to negotiate lower drug prices. But the senator was unavailable to meet the advocacy group for those over 50 years old, which claims over 900,000 members in Arizona, and instead the organization met with Sinema's staff.
AARP Arizona state director Dana Kennedy said she typically works with the senator's staff, "but [on] something like this, we should hear directly from the senator."
"We do hope she supports allowing Medicare to negotiate because her constituents certainly support it," Kennedy said in an email. "She ran on lowering Rx costs and that is how you do it AND it saves Medicare BILLIONS of dollars."
Sinema's aides though, says they have been public about her stance on the issue and supports other drug pricing proposals, pointing to comments she made recently to the Arizona Republic saying that Congress "should be focused on policies that ensure prescription drugs are available at the lowest possible cost."
Depending on how things turn out, activists say there will be support for a primary challenge in four years.
Marilyn Rodriguez, an Arizona progressive strategist at Creosote Partners, said she has seen polling for Arizona Democratic Reps. Greg Stanton and Ruben Gallego, Tucson Mayor Regina Romero and Arizona Superintendent of Public Instruction Kathy Hoffman showing them to all be credible challengers against Sinema.
"I don't think that we are at the point where Arizona has given up on Kyrsten Sinema," said Rodriguez. "But I do think that we are on the brink."
Nuestro PAC, a Latino mobilization group, has already launched a "Run Ruben Run" campaign, urging Gallego to challenge Sinema in the primary.
"When he gets done running for Congress [in 2022], if he wants to run for the Senate, I'll have a bunch of money to give him and a nice email list," said Nuestro PAC president Chuck Rocha.
Asked if he believes she could lose a primary, Gallego told CNN: "I think there's a lot of time between now and 2024, a lot of time for the senator to put things right with voters in Arizona. And let's hope she does it."