Congress will return from its summer recess later this month, and some Democrats are already gearing up for a political fight – with each other.

Democratic lawmakers are looking to pass their $3.5tn spending package, after the House and the Senate approved the blueprint for the budget bill last month. The ambitious legislation encompasses much of Joe Biden’s economic agenda, including proposals to expand access to affordable childcare, invest in climate-related initiatives and broaden Medicare coverage.

But to get the bill passed, Democrats will first need to reach an agreement on the cost of the legislation. Centrist Democrats, including Senators Kyrsten Sinema and Joe Manchin, have expressed concern about the bill’s $3.5tn price tag, while progressives have indicated they will fiercely oppose any attempt to cut funding in the proposal.

With his entire economic agenda hanging in the balance, Biden will need to convince the two fractious wings of his party to come together and pass a comprehensive spending package. And given Democrats’ extremely narrow majorities in both the House and the Senate, there is virtually no room for error.

Despite warning signs of intra-party friction over the cost of the budget bill, congresswoman Suzan DelBene, who chairs the centrist New Democrat Coalition, said the House’s focus right now should still be on the content of the legislation.

“I think discussion of a number is more distracting when the focus really needs to be on, what is the substance going to be of this legislation?” DelBene told the Guardian. “If we have strong legislation the people support, I think we can find the path forward.”

Over in the Senate, majority leader Chuck Schumer is attempting to advance the bill using reconciliation, meaning Democrats do not need any Republican support to pass the legislation. But the 50-50 split in the upper chamber means that every single Democratic senator must be on board to get the bill approved.

Schumer has been clear-eyed about the challenges ahead for the legislation. Shortly after the Senate approved the blueprint for the bill in a party-line vote last month, Schumer told reporters, “We’ve labored for months and months to reach this point, and we have no illusions – maybe the hardest work is yet to come.”

Manchin proved Schumer’s point last Thursday, when he wrote a Wall Street Journal op-ed calling for a “strategic pause” in advancing the spending package.

“While some have suggested this reconciliation legislation must be passed now, I believe that making budgetary decisions under artificial political deadlines never leads to good policy or sound decisions,” Manchin said in the op-ed. “I, for one, won’t support a $3.5tn bill, or anywhere near that level of additional spending, without greater clarity about why Congress chooses to ignore the serious effects inflation and debt have on existing government programs.”

Bernie Sanders, the leftwing chairman of the Senate budget committee, responded to Manchin’s warning in kind, threatening to torpedo the bipartisan infrastructure bill if the spending package is not approved.

“Rebuilding our crumbling physical infrastructure – roads, bridges, water systems – is important,” Sanders said on Twitter. “Rebuilding our crumbling human infrastructure – healthcare, education, climate change – is more important. No infrastructure bill without the $3.5tn reconciliation bill.”

Progressive groups have echoed Sanders’s argument, insisting that every component of the $3.5tn legislation is vital. Sanders had initially called for spending $6tn on the budget bill, so progressives already view the current price tag as a concession.

“We’re in a moment of crisis. Is this really the time for the Senate to press pause?” Ellen Sciales, the communications director of the climate group Sunrise Movement, said in a statement.

She added: “If the Senate can’t pass an incredibly popular climate and jobs plan during a summer of unprecedented, fatal climate disasters, and an economy reeling from a global pandemic, we must abolish the Senate. $3.5tn was the compromise.”

Natalia Salgado, the director of federal affairs for the Working Families Party, noted that some progressive economists have suggested the US needs to spend $10tn over 10 years to meet its obligations in the Paris Climate Agreement.

“We’re going to come nowhere near that,” Salgado said. “So we can’t afford to lose a single cent in this $3.5tn. Every single penny will count.”

Despite the war of words between moderates and progressives, the White House has continued to express confidence that Congress will ultimately reach an agreement on the legislation.

“The president and his whole team are proud of and fighting for the substance of his Build Back Better agenda,” a White House official said in a statement. “These are complex processes, but as recent weeks have demonstrated, leaders in Congress and the President know how to move them forward.”

And DelBene similarly said that her group, which represents 95 Democrats in the House, remains committee to advancing both the bipartisan infrastructure bill and the spending package.

“The strength of the legislation, both on the infrastructure side and the reconciliation bill, is what people are going to look at moving forward,” DelBene said. “I think we want to see the infrastructure deal and the reconciliation bill get done.”

The risk of the strategy is that, in trying to pass everything, Biden and congressional Democrats may end up with nothing.