As the Senate headed into hours of amendment votes on a $900 billion economic stimulus bill Thursday evening, bipartisan negotiators were furiously working to find as much as $100 billion in cuts even as they prepared to add up to $25 billion in infrastructure spending.

It was unclear whether the Senate would be able to pass the measure Thursday night, but Majority Leader Harry Reid (D-Nev.) said earlier in the day that he would like to do so, with or without the compromise amendment. Senators said Democratic leaders were giving them assurances that if the amendment was not ready in time for a final vote Thursday night, the changes they are seeking could be made during conference talks with the House.

“If this package isn’t improved here, but assurances were given before conference was completed that additional changes would be made. That’s another possibility. There are conversations going on,” said Sen. Kent Conrad (D-N.D.), who has been highly critical of the bill’s ability to stimulate the economy as currently written.

And Democratic aides raised the specter of Reid using procedural tactics to bring debate on the bill to a close. He could do that late tonight, setting the state for a possible vote Saturday.

Still, a bipartisan group of Senators working to trim the bill’s price tag said they hoped to finish their amendment tonight or tomorrow, and many Senators indicated they would have a difficult time voting for the bill without cuts. They said they planned to continue vetting the amendment during their hours on the floor voting.

Sen. Ben Nelson (D-Neb.), one of the leaders of the group of 18 Senators, declined to say how much money the amendment might seek to cut, but he acknowledged, “It is sizable and we’ve continued to find areas of adjustments.” Sen. Jon Tester (D-Mont.) said the figure was in the “ballpark” of $100 billion.

However, the group has also begun to discuss adding more money for infrastructure projects that would create construction jobs around the country.

“It’s premature to say that that’s in, but we have made room for more infrastructure in it,” said Nelson.

Though Senate Democratic leaders have viewed the amendment with suspicion because the group has targeted some education funding for cuts, the White House appears firmly behind the effort.

Nelson said President Barack Obama is “very supportive of the effort. They understand that a bipartisan effort is important and getting a bipartisan result is something that they really prefer.”

And Democratic leaders have privately acknowledged that they might not be able to garner a filibuster-proof 60 votes in order to pass the bill without the proposal. Publicly, Reid has asserted that he believes he has the 60 votes he needs.