Having averted a government shutdown and potential default this month, Democrats in the US Congress move on to the even harder task of passing President Joe Biden's $1.75 trillion domestic investment bill.

After months of intraparty dickering, moderate Democratic Senator Joe Manchin was withholding support for the sweeping bill to expand social programs and invest in technology to battle climate change. His stance complicates Senate Majority Leader Chuck Schumer's goal of approving it before Christmas, 10 days away.

"It's going to be close," Biden said on Wednesday when asked if he hoped to see the legislation completed by the end of the year.

Manchin's office had no comment when asked if the senator was expected to talk to the president on Wednesday.

"The president's been speaking with Senator Manchin and I look forward to hearing about further progress," Schumer said on Tuesday, sticking to his timetable but not guaranteeing success.

Schumer, Manchin and Senator John Tester, also a centrist, were spotted meeting in the Capitol on Wednesday. With no votes to spare in passing Biden's  "Build Back Better" initiative amid solid Republican opposition, Schumer faces his biggest challenge since becoming majority leader in January. He is feeling the heat from all sides.

His top lieutenant, Senator Dick Durbin, has been clamoring for a showdown that would force Manchin to either fall into line and vote for the BBB bill or defy Biden in a way that could severely damage his presidency and Manchin's own party. Asked whether Schumer should schedule votes on the bill without nailing down victory, Durbin said:  "It's always a risk," adding:  "Many people will sit on the fence as long as possible.