Senate Democratic and Republican leaders were digging in Wednesday for what could be an ugly political collapse of bipartisan efforts to pass comprehensive immigration reform, with both sides already blaming their potential failure to pass legislation on the other’s unwillingness to compromise.

Senate Majority Leader Harry Reid (D-Nev.) announced Wednesday afternoon that he would defy GOP filibuster threats and attempt to force last year’s Senate-passed immigration reform package to the floor next week.

Reid said that bill will act as a “framework” for the two-week debate and that he was still open to allowing the members of the bipartisan immigration negotiating group to offer any deal they reach as a substitute amendment for the underlying bill. “We all realize that bill is imperfect. But it’s the place we’re going to start,” Reid said.

But he rejected calls for more time to allow a bipartisan deal to emerge. “Some have said to me … ‘do it another time, give us a little more time.’ We don’t have anymore time,” Reid said.

That prompted an angry response from Republicans, who accused Reid of attempting to marginalize a months-long bipartisan effort to draft compromise legislation.

Senate Minority Leader Mitch McConnell (R-Ky.) rejected the idea on the Senate floor and warned Reid that it would scuttle the bipartisan effort. McConnell said he was concerned Reid was “brush[ing] aside in favor of last year’s unsuccessful bill. … I strongly urge my colleagues to reconsider this approach, if indeed it’s the one they plan to take … any effort to move legislation on this issue that isn’t the result of the ongoing bipartisan discussions would be a clear signal from Democrats that they are not yet serious about immigration reform.”

Senate Republican Conference Chairman Jon Kyl (Ariz.), McConnell’s point man on immigration reform, also said he was concerned “by reports that Senate Democrats may attempt to pass last year’s immigration legislation rather than continue working toward a bipartisan solution.”

All 23 Republicans who voted for last year’s immigration bill sent a letter to Reid on Wednesday warning they would not vote for that measure again and calling on the Majority Leader to allow more time to work out a bipartisan deal.

The 2006 bill “is dead. We need a new bill,” said Sen. Lindsey Graham (R-S.C.), who added that “we need a product coming off the Senate floor with 75 to 80 votes. And that product is within reach.”

With the procedural showdown threatening to derail movement of a bill, both sides acknowledged that the next 24 hours in the Senate will largely determine whether Congress can craft a bipartisan bill this year. But neither Democrats nor Republicans could predict how it ultimately will shake out, and the level of optimism continues to vary widely depending on whom you ask.

Sen. Bob Menendez (D-N.J.) said lawmakers will have to see how each party’s leadership decides to proceed, and the outcome of the next round of bipartisan talks, which were set to begin again Wednesday afternoon.

“The bottom line is we need to move here,” Menendez said. “If we don’t take advantage of these two weeks, we will lose our opportunity. It will be hard to reschedule.”

Senators in both parties concede that the final two weeks of May could be the chamber’s only chance to reach a deal this Congress. Momentum is on their side heading into the Memorial Day recess, and too much legislation is competing for calendar space when Senators return to work next month.

Sen. Mel Martinez (R-Fla.) said early Wednesday that Senators are “at a critical moment” in the discussions, and while some may believe the negotiations are falling apart, he wasn’t ready to give up.

“The next 24 hours or so will tell us where we are,” Martinez said. “We need to recommit to the process of a bipartisan solution.”

Sources indicated that McConnell was going to survey his Conference to see whether he could garner GOP unanimity in blocking Reid from moving forward with debate on the old bill next week.

GOP leaders want to wait until the bipartisan group of about 10 Senators produces a consensus bill before moving to a floor debate. Alternatively, they want Reid to use as a placeholder a “shell” bill that contains little more than immigration-related titles to kick off the Senate discussion.

Graham, who has credited Reid with forcing Senators to address the issue on a deadline, wasn’t ready to say the issue would be decided by the end of today, but he did acknowledge that time is running short. By next Monday, Graham said lawmakers would know whether a deal can be reached and if immigration can get done this year.

“We’ll know in the next couple of days,” Graham said. “I say it’s 50-50. How it ends, I don’t know.”

Graham added that if the bipartisan talks bear fruit, he believes their measure would be overwhelmingly popular, garnering 75 to 80 votes.

“This is the best chance there will be for immigration reform in years,” he said. “This is it.”