In case you doubted that House progressives would stick to their strategy in the intraparty battle over President Biden’s agenda, they just reiterated their intention to vote against the $1 trillion bipartisan infrastructure bill on Thursday, to pressure the Senate to complete the much bigger social policy bill

“We will only vote for the infrastructure bill after passing the reconciliation bill,” declared a statement from Rep. Pramila Jayapal, the chair of the Congressional Progressive Caucus. The reconciliation bill is the multitrillion-dollar social infrastructure bill that is meant to pass the Senate by simple majority.

This posture is provoking frustration from moderates and centrists. Many of them appear to understand this standoff as a conflict between themselves and progressives, each side pushing and pulling to get their preferred outcome.

But while there’s some truth to this, in another sense the most serious problem facing the centrists is not the progressives. It’s their ideological counterparts in the Senate, Sens. Joe Manchin III (D-W.Va.) and Kyrsten Sinema (D-Ariz.).

That’s because the Terrible Two are the main impediment to arriving at a reconciliation bill everyone can live with. It’s likely that virtually all House moderates and centrists would support a very robust reconciliation bill with ambitious provisions on climate change, child care, health care, education and many other things, provided last-minute disagreements are ironed out.

But Manchin and Sinema seem to be another matter entirely. No one knows what they’ll accept from a reconciliation bill. If they were to be very clear on what they could accept, and persuasively demonstrate that they will vote for it even if the infrastructure bill passes first, it might be easier for progressives to agree to do that.

Which points to another big irony here: If this process implodes, the Democrats who represent swing districts may well suffer most. Passing a broad, popular agenda may not stop Republicans from picking up seats in the 2022 midterms, but if Democrats don’t pass that agenda, they may well get swamped in a wave election.

Who loses in a wave election? Not the progressives who represent safe Democratic districts. It’s the ones representing swing districts, often called “frontliners.”

Rep. Katie Porter (D-Calif.) understands this well. She is both a member of the Congressional Progressive Caucus and a frontliner in a swing Orange County district. Porter told us that vulnerable frontliners, too, have a great deal to gain substantively and politically from passing a robust reconciliation bill.

Porter noted that many provisions in the infrastructure package are long-range projects that won’t even be underway by the midterms. By contrast, many provisions in the reconciliation bill will have an immediate impact, such as expanded Medicare eligibility and home care services and assistance for child care.

“Those are things that will immediately begin to improve the lives of Americans and will begin to immediately improve our economy,” Porter told us. “Democratic members, regardless of your district’s composition, this is what voters want.”

Porter said many frontliners are adamant that “we cannot fail” and that “we need to deliver the president’s entire agenda.”

Porter also told us she will vote against the infrastructure bill Thursday “if there’s not a framework and an agreement on how we move forward.”

It’s increasingly likely that if the infrastructure bill is to pass, it will only be if the two main Senate Democratic centrists can reassure progressives in some way, by agreeing to a reconciliation framework that isn’t too whittled down and by demonstrating a durable commitment to it.

“We need Manchin and Sinema to tell us what they’re comfortable with,” Porter told us.

A lot of progressives appear to agree. A Democratic aide who was on a conference call with Congressional Progressive Caucus members on Tuesday told us that more than two dozen members voiced support for holding the line.

For the party as a whole, it’s all or nothing. If they fail, not only will their whole agenda lie in ruins, but Biden’s presidency will be wounded and it could be years before they regain majorities and govern again.

The big question now is whether House Speaker Nancy Pelosi (D-Calif.), with Biden doing his part, can produce a general framework on reconciliation that everyone can live with, enabling progressives to help pass the infrastructure bill Thursday. If not, and progressives hold the line, Pelosi may have to postpone the vote.

But ultimately, here’s what we know: For a Thursday infrastructure vote to succeed, at a minimum it will require Manchin and Sinema to step up. House centrists might devote some energy to making that happen.

It’s all very touch and go. But that — plus passing a continuing resolution to keep the government open and forestalling a debt default — would allow Democrats to stick the landing on a legislative tumbling run worthy of Simone Biles.

Then they can say they did something extraordinary for the country — and kept alive their hopes of retaining their majorities.

Because they’re all in it together, it’s the only real choice they have. Nobody should understand this better than the moderates and frontliners. Now if only they could prevail on Manchin and Sinema to do their part.