For a politician at the center of Democrats’ agenda, Sen. Kyrsten Sinema (D-Ariz.) is uncharacteristically quiet. Her vote could make or break Democrats’ social spending/climate package, and she’s negotiating what she’ll vote for with only a handful of lawmakers and White House aides, report The Washington Post’s Seung Min Kim and Ashley Parker.

That’s frustrating her constituents, some of whom feel shut out of the process. Citing her lack of transparency, a group of veterans on her advisory council recently quit, saying Sinema is an obstruction to progress. “Nobody knows what she is thinking because she doesn’t tell anybody anything,” one of the veterans who derided Sinema, Sylvia González Andersh, told the New York Times. “It’s very sad to think that someone who you worked for that hard to get elected is not even willing to listen.”

In part because Sinema’s vote is so critical to major government policy changes, staying quiet about what she likes or dislikes is also arguably a disservice to public discourse. The more people know about what she and other lawmakers are debating, the more Americans can participate in shaping legislation that will affect their lives.

So, why is Sinema so mum amid all this criticism? Let’s review some possible theories.

Her office says she doesn’t negotiate through the press. “Sen. Sinema does not negotiate policy details through the press,” her spokesman, John LaBombard, said in a statement.

What does he mean by that? Reading between the lines, it appears that Sinema has decided that the media is not helpful to getting what she wants out of legislation.

Other politicians — most politicians — feel very differently. They’re talking to journalists, they’re writing op-eds, they’re getting in front of TV cameras, they’re on social media. Sen. Bernie Sanders (I-Vt.) — who has been frustrated by Sinema and the other Democratic Senate holdout, Sen. Joe Manchin III (D-W.Va.) — is a good example of this. He sees media as a tool to make his case for what should be in the bill and build support for it.

Sinema appears to see media not as a tool, but as an obstacle to progress.

She doesn’t get pinned down on a specific position. In negotiations this fluid, everyone’s going to have to give up some of what they want. Lawmakers who stake their position publicly can get burned when they have to give up that position.

The leader of the Congressional Progressive Caucus, Rep. Pramila Jayapal (D-Wash.), was pretty adamant for months that liberals want $3.5 trillion in social safety net spending. “$3.5 trillion was the smaller number,” she said in September, when asked if she’d be willing to lower it.

Now, her group will have to vote for a lot less if it wants anything at all. (As the number has come down closer to $2 trillion, Jayapal has stressed her position has “never been about the price tag. It’s about what we want to deliver.”)

By refusing to state her positions publicly, Sinema doesn’t face the same kind of potential embarrassment.

(The only time Sinema stated her position on this social spending bill publicly is when she said she wouldn’t vote for anything that costs as much as $3.5 trillion. And she and Manchin appear to have won that battle.)

She doesn’t think it’s fair to those she’s negotiating with. Sinema doesn’t want to “disrupt sensitive negotiations with other senators or the White House,” Kim and Parker report.

The norm in Washington is for negotiators to talk at least somewhat publicly about what’s happening behind the scenes — including President Biden, who was remarkably candid during a CNN town hall this week, including about Sinema’s positions.

But Sinema appears to feel that saying so much as a word publicly is a betrayal both to her and to those she’s talking to.

Her approach is reminiscent of a much more old-school style of legislating, when powerful lawmakers huddled together and cut deals. (Sinema’s critics would deride that as a return to the proverbial smoke-filled backrooms, where there was little to no transparency about lawmaking and special interests ruled the day.)

It’s more difficult to align her stance with outside groups. Among the theories about why Sinema stays quiet, this is among the most cynical. But it’s one her critics are elevating.

Rep. Ro Khanna (D-Calif.), a big critic of Sinema, told Kim and Parker that Sinema’s skepticism to raising corporate and individual taxes to pay for the bill rings hollow, given she voted against those tax cuts when Donald Trump was president.

“It makes you wonder,” Khanna said, “what are the special interests that are driving that decision? It’s obviously not conviction, because she voted against the tax cuts in the first place.”

Sinema is similarly skeptical of a prescription-drug-pricing measure to help pay for the bill, perplexing Democratic negotiators because it’s popular with voters. Her critics point out that Sinema is one of the top recipients in Congress of donations from the pharmaceutical industry, Kaiser Health News reports. That includes about $25,000 from that industry over the summer.

Being controlled by special interests is a common attack lobbed at politicians, and it’s often hard to prove or disprove. Sinema is probably avoiding such attacks on a dozen other negotiating points that we don’t know about because she has kept her views so private.

She talks about what she supports and doesn’t after the bill is voted for. This is a strategy Sinema followed after a bipartisan infrastructure bill passed the Senate this summer that she helped negotiate. As senators reached a deal, they held a news conference, where Sinema answered questions about it and talked a lot — for Sinema, at least — about private negotiations.

“It came not because folks were interested in their bridge, or road, or piece of the middle mile of broadband,” she said. “It came because each of us are committed to demonstrating to the country and the world that our government can work for the people of the United States of America.”

On one hand, this strategy lets Sinema claim victory when it’s all done. But on the other, she’ll have no proof that what she wanted ultimately got in the bill, because she’s stayed so quiet throughout this process.