Senate Democrats are racing to pledge their support for a single-payer health care bill set to be introduced by Sen. Bernie Sanders (I-VT) this week. But in conversations with lawmakers, the momentum that seems so evident behind the bill is complicated by fear the party may not be honestly reckoning with other political realities.

Roughly a third of the Senate’s Democrats have announced their support for Sanders’ Medicare for All bill, which is set to be introduced on Wednesday. It’s a remarkable swelling of support for the concept—one that exemplifies Democrats’ evolution into a more progressive, populist party in the age of both Donald Trump and, to a certain degree, Obamacare. A Vermont Democratic Socialist has become the primary policy driver of a political party to which he does not belong.

But the likelihood of Sanders’ bill passing remains not just slim, but negligible. And there is mounting concern within Democratic leadership that the expectation for legislative action could outpace their comfort in pushing such a proposal.

Take Senate Minority Leader Chuck Schumer’s noncommittal reaction to the marquee issue: “There are many different bills out there. There are many good ones,” he said on Tuesday.

Support for Sanders’ bill has come, primarily, from the progressive corner of the Democratic Party, where single-payer has long been considered the platonic ideal for a national health care system. Sens. Sheldon Whitehouse (D-RI), Jeff Merkley (D-OR), Ed Markey (D-MA), Tammy Baldwin (D-WI), Mazie Hirono (D-HI), and Brian Schatz (D-HI) have all signed on to the idea.

But some of the momentum has clearly been fed in part by the 2020 presidential campaign jockeying, too. Sens. Kamala Harris (D-CA), Kirsten Gillibrand (D-NY), and Cory Booker (D-NJ) all notably announced their support. Oft-discussed candidates Sens. Elizabeth Warren (D-MA) and Al Franken (D-MN) did, too, though their long-standing progressive bona fides place them in the former category as well.

Eight years ago, such support would have been unheard of. When the Obama administration held its first public health care hearings in 2009, it was an open question whether a single-payer advocate would even be invited. Now any Democrat with a desire to run for the White House, and a majority of Democrats in the House, support the concept.

But while the momentum behind Sanders’ bill is undeniable, so too is the fact that a large swath of Democratic senators remain skittish, opposed, or simply silent on the proposed legislation. Sen. Tom Carper (D-DE), who was a leading voice over the summer in convincing Republicans to abandon repeal-and-replace in favor of bipartisan solutions, urged caution in backing a largely untested health care system and applying it nationwide.

“I believe that states should be laboratories of democracy,” Carper told The Daily Beast. “The idea that states could experiment in single-payer systems—something along those lines—that the rest of us can learn from before we take something like that and do it on a national level, I think that makes sense.”

Sen. Joe Manchin (D-WV) told The Daily Beast that he was “skeptical that single-payer is the right solution,” though he said he wants to “carefully consider all of the options.” Sen. Claire McCaskill (D-MO) has stated her opposition. And Sen. Heidi Heitkamp (D-ND), in a statement, encouraged her fellow lawmakers to build off the market-based system that defined Obamacare, implying that single-payer was a bridge too far.

These skeptics all have one thing in common: They are up for re-election. And, save Carper, they are all from moderate-to-conservative states.

Because of that, champions of Sanders’ bill have argued that the senators’ opposition to single-payer is politically driven, sparked by a flawed belief that voters in “red states” will recoil at the idea of government-run health care. That former Sen. Max Baucus (D-MT) announced his support for single-payer this week—after helping derail the idea of a public option during the crafting of Obamacare—was treated as proof those current Democratic senators who were acting squeamish were doing so because of re-election concerns.

But while Baucus may now be open to single-payer, other Democrats now out of office are not. Asked if he supported the concept, former senator Ben Nelson (D-NE) replied with a one-word email: “No.” Former senator Kent Conrad (D-ND), who was among three Democratic senators in 2009 to vote against two public-option proposals in the Senate Finance Committee, also had reservations.

“I think it is unlikely we will get single payer in the foreseeable future because the interests arrayed in opposition are too strong and the cost to the federal government is too high,” he told The Daily Beast.

The Democratic Party of today is wildly different from the one that existed when Obamacare was put together. It is, undoubtedly, far more progressive. Its elected ranks are also far smaller.

Within this political context, support for Sanders’ bill has grown and the Overton window of the debate over health care has irrevocably shifted. Currently, the compromise proposals being proposed by Democrats are the progressive ideals that were scrapped during the passage of Obamacare—such as Sens. Chris Murphy’s (D-CT) and Sherrod Brown’s (D-OH) proposals for Medicare buy-in measures.

“It moves the conversation more to the left, which you know puts pressure on Republicans to come up with an alternative,” Conrad said of Sanders’ bill. “And the alternative might be some variation on the status quo or some other alternative they so far have not been able to come up with.”

The question facing Democrats is whether the Sanders approach can help expand their ranks or shrink them. Republicans are already gearing up to capitalize on the Medicare for All push, though the chairman of the Democratic Senatorial Campaign Committee—a group charged with electing Democrats to the Senate and supporting the party’s incumbents—brushed off those concerns.

“Look, I encourage everybody to support legislation that they think is important for the country. And everyone's going to be taking a close look at the proposals that are out there,” Sen. Chris Van Hollen (D-MD) told The Daily Beast. “No, I don’t think it hurts the party’s chances. There are lots of ideas out there.”