South Korean President Moon Jae-in (L) listens as Speaker of the House Nancy Pelosi speaks during a press conference after a meeting on Capitol Hill on May 20, 2021 in Washington, DC., the United States. Photo: VCG

US House Speaker Nancy Pelosi expressed confidence a massive infrastructure bill will pass this week but acknowledged it might not get a Monday vote as planned, with fellow Democrats warning critical work remains to meet the party's deadlines.

Democrats have been scrambling to hammer out a landmark plan to upgrade the nation's roads and bridges, but are also under immense pressure to finalize a $3.5 trillion public investment package and fund the government to avert a looming shutdown - all by September 30.

The week is among the most critical of President Joe Biden's tenure, with opposition Republicans digging in against his Build Back Better program that would invest in climate change policy, lower childcare and education costs for working families and create millions of jobs. But Pelosi, despite her confidence that the $1.2 trillion infrastructure bill that has already cleared the Senate with bipartisan support will pass the House of Representatives "this week," hinted at potential quicksand ahead.

"I'm never bringing a bill to the floor that doesn't have the votes," the top Democrat in Congress told ABC Sunday talk show This Week, asked about whether she will bring the infrastructure bill to the floor Monday as previously agreed. "It may be tomorrow - if we have the votes," she said.

"You cannot choose the date," she added. "You have to go when you have the votes, in a reasonable time. And we will."

Biden told reporters on Sunday he was "optimistic" Pelosi would get the agenda through the house this week, adding "it's going to take the better part of the week."

Pelosi told her Democratic colleagues in a letter Saturday that they "must" pass both of Biden's huge spending bills, along with legislation that keeps the federal government operating into the next fiscal year beginning October 1.

"The next few days will be a time of intensity," she wrote.

Pelosi is running into not only a buzzsaw of opposition from Republicans; Democratic progressives and moderates have made clear they need to see quickly exactly what goes in the $3.5 trillion bill.