Nancy Pelosi appeared determined to press ahead with a make-or-break vote on Joe Biden’s $1.2tn bipartisan infrastructure bill in the US House of Representatives on Thursday, even as progressive lawmakers threatened to sink the flagship piece of the president’s legislative agenda.

“We are proceeding in a very positive way to bring up the bill . . . in a way that can win,” Pelosi, the Democratic speaker of the House, told reporters on Capitol Hill on Thursday morning, after several days of crunch talks with the White House and members of Congress.

Biden has staked his presidency on two pieces of legislation: the bipartisan infrastructure bill and a $3.5tn investment in America’s social safety net that is staunchly opposed by Republicans and would need to pass the Senate with only Democrat votes via a manoeuvre known as reconciliation.

The larger bill has been held up by opposition from two Democratic senators on the right of the party, Joe Manchin from West Virginia and Kyrsten Sinema from Arizona.

On Thursday, Manchin revealed he had told Democratic leaders he could not guarantee his support for a reconciliation bill larger than $1.5tn — considerably lower than Biden had hoped. Progressive Democrats in the House have told Pelosi they will not vote for the bipartisan infrastructure package without clarity on the second, larger spending bill.

After meeting with the Speaker on Thursday afternoon, House progressive caucus chair Pramila Jayapal told reporters progressives were “in the same place”, adding: “We will not be able to vote for the infrastructure bill until the reconciliation bill has passed.”

Pelosi, a longtime legislator with a record of emerging victorious from tough negotiations, has repeatedly said she will not hold a vote until she is confident she will win. Yet despite the Speaker’s optimistic tone on Thursday, many on Capitol Hill remained sceptical that she would be able to get the infrastructure package over the line this week.

When asked by reporters whether he was confident the infrastructure bill would pass on Thursday, Steny Hoyer, Pelosi’s number-two, said: “Nope.”

Progressives cried foul late Wednesday after Manchin issued a lengthy statement saying he could not support $3.5tn in additional spending. “At some point, all of us, regardless of party must ask the simple question — how much is enough?” the senator asked.

Manchin has proven to be a thorn in the side of the White House and many members of his own party in a Senate that is divided, 50-50, between Democrats and Republicans. Kamala Harris, the vice-president, can cast a tiebreaking vote, but any one Democratic senator can torpedo a bill given the tight arithmetic.

Pelosi nevertheless insisted on Thursday that a reconciliation bill would eventually be signed into law, calling the proposed sweeping social investment a “culmination of my service in Congress”.

The mad dash to salvage Biden’s domestic agenda comes as lawmakers scramble to avert a government shutdown and confront the spectre of a potential US debt default should lawmakers fail to raise the federal borrowing limit.

Both the Senate and the House are expected to pass a “continuing resolution” later on Thursday that would fund the government for another two months and avoid a shutdown ahead of a Friday 12:01am deadline.

But the debt ceiling issue remains unresolved, with Republicans refusing to sign on to raising the borrowing limit and Democrats insisting that they do not have enough time to pass the measure without opposition support.

Treasury secretary Janet Yellen warned earlier this week that the government risks running out of money by October 18 — an outcome she warned would be “catastrophic” and precipitate a financial crisis. On Thursday, Yellen told lawmakers that she would support eliminating the debt ceiling altogether.

Additional reporting by Colby Smith in New York