Progressives are furious at Sen. Kyrsten Sinema, whose reasons for opposing Biden's $3.5 trillion agenda are mystifying

• Progressives are increasingly frustrated by Sen. Sinema's refusal to support the "Build Back Better" bill.
• Some argue there are no political reasons for Sinema to obstruct or shrink the legislation.
• Sinema has refused to lay out what she'd like to see in the social-spending bill Biden is pushing.

Progressive lawmakers and pundits are increasingly enraged by Sen. Kyrsten Sinema's refusal to support President Joe Biden's $3.5 trillion "Build Back Better" bill, and some argue there's simply no reason for Sinema to obstruct her party's signature legislation.

While the Arizona Democrat has expressed opposition to the size of the bill and it's tax provisions, she's refused to lay out what she'd like to see in the legislation, which features universal pre-K, paid family leave, tax increases on the rich, and climate priorities, among other major Democratic agenda items. She hasn't been forthcoming about her position with her colleagues or the public and has only made vague comments concerning her multiple discussions with the president.

When asked by a reporter where she stood on the bill on Wednesday, Sinema mocked the question, replying that she's "in the Senate" and "right in front of the elevator."

Many Democrats insist there's little political logic behind Sinema's opaque opposition. Pollster Nate Silver called Sinema's obstruction "pretty irrational" as it damages her relationship with Arizona's Democratic voters.

Tom Perriello, a former Democratic congressman from Virginia who lost his seat after voting for the Affordable Care Act in 2010, argued in a Washington Post op-ed on Wednesday that even purple state Democrats like Sinema can safely vote for the social-spending bill. He pointed to polling that shows the legislation enjoys popular support, and noted that the opposition effort from lobbyists and Republicans isn't particularly strong. He wrote that both Democratic and independent voters will punish the party if they don't pass the sweeping social policy reform Biden promised.

"The middle and working classes want to see the agenda of the president they elected become law, and will remember if it fails," Perriello wrote.

While Sinema barely eked out a win in 2018 with just over 50% of the vote, she's now facing bleak favorability numbers among Arizona Democrats over her moderate policy stances.

Rep. Katie Porter, a California progressive from a swing district, argued on Wednesday that Sinema is failing her constituents by refusing to make clear what she wants to see in the bill.

"Until Senator Sinema stops being cute, and starts doing her job and leading for the people of Arizona, we're simply not going to be able to move the president's agenda forward," Porter said, adding, "I was not elected to read the mind of Kyrsten Sinema. Thank goodness, because I have no idea what she's thinking."

Progressives have expressed somewhat more confidence in West Virginia Sen. Joe Manchin, who's also negotiating with the White House but appears dedicated to passing a smaller version of the bill. Unlike Manchin, who represents a very conservative state, Sinema could be replaced by a more progressive Democrat as Arizona moves to the left. Indeed, her fellow Arizona senator, Democrat Mark Kelly, is publicly supportive of the bill, and progressive activists and donors are already organizing to primary Sinema in 2024.

Frustration mounted this week when news broke that Sinema would hold a fundraiser for business lobbyists who oppose much of the bill.

Rep. Ro Khanna, a California progressive, on Wednesday called Sinema "an enigma" and said her obstruction of the legislation's passage is "unconscionable." Khanna wouldn't speculate about Sinema's motivations for opposing the legislation, but said Sinema's fundraiser at least amounted to bad "optics."

"It is just remarkable to me that we have let the situation get to this point that a single first-term senator from a state that the president carried ... is holding up the entire Democratic agenda, holding up benefits for the working class and middle class," Khanna told MSNBC.

Liberal pundits are also ratcheting up the pressure on Sinema.

"I have never seen a Democratic senator behave anything like that. Not once. Not ever," MSNBC host Lawrence O'Donnell said Wednesday night. "There is no playbook for how to negotiate with a senator who talks like that."