(Bloomberg) -- Senate Democrats are expressing confidence they can pass President Joe Biden’s Build Back Better agenda before Christmas, but one key holdout -- Senator Joe Manchin -- said he’s “skeptical” he can vote for a bill that includes major economic changes.

Many Democrats say they can meet Senate Majority Leader Chuck Schumer’s deadline for passing the bill by Dec. 25, which is key to preventing an interruption in the monthly child tax credit payments that the party considers critical for economic and political success.

“We’re down to just a small number of issues. We’re all here. The people who are intensely involved in those are negotiating almost around the clock,” Senator Elizabeth Warren, a Massachusetts Democrat, said Tuesday. “Another month, another three months is not going to make this any easier. It’s time for us to cut our final deals.”

Manchin, a West Virginia Democrat, said Monday that he’s not so sure. Elements of the roughly $2 trillion spending package would have significant impact on the economy and warrant more study, he said. Manchin’s vote in the 50-50 Senate is critical for Democrats to pass legislation that isn’t expected to get any support from Republicans.

“We’re talking about making major changes in our tax code. We’re talking about major overhaul of our social reforms. And we’re talking about a tremendous overhaul of our climate positions that we have,” Manchin said. “I’ve always said there’s a balance between economy and environment.”

Democrats are worried that momentum on the bill could stall if it isn’t finished before the end of the year, delivering a major political blow to Biden and the party heading into a congressional election year. But Manchin isn’t the only obstacle to quick passage. 

Democrats are still working with the Senate parliamentarian to ensure the bill complies with the chamber’s rules for reconciliation, a process that allows the economic package to pass with only Democratic votes. It’s not clear how much longer that process will take.

Schumer told all Senate Democrats in a Monday letter that formal meetings with the parliamentarian to discuss key tax and health care provisions have not yet been set. The parliamentarian is already weighing a plan to provide deportation protections to nearly 7 million undocumented immigrants, but hasn’t yet issued a determination.

Senator Tim Kaine said Democrats must pass the bill before Jan. 15, the date that the first monthly child tax credit payment of up to $300 per child would go out under the new bill. The last payment under current law will be distributed Dec. 15.

“We got to pass it sufficiently in advance of January 15 to make sure that families continue to get this child tax credit.” Kaine told reporters. “If you get too close to January 15 without an answer then there’s a delay or a risk of missing a month.” 

Without congressional approval, the child tax credit would decrease in size to $2,000 per child down from as much as $3,600. The monthly payments would also stop and parents would only be able to claim the money at tax time, likely causing disruptions for parents who have become accustomed to the monthly payments to cover expenses throughout the year.

Senator Sherrod Brown, an Ohio Democrat who helped develop the expanded child tax credit, said the payments could help buoy Democrats as they run for re-election next year.

“The problem so far as to politics is the voters don’t really kind of know where this came from. They’re getting this check every month in their direct deposit or in their mailbox,” he said. “But after this bill’s signed, Democrats are going to be talking who’s side are you on so that’s why all these predictions of Republicans winning next year are specious.”

©2021 Bloomberg L.P.