Shelby, the ranking member of the Senate Banking Committee, said he wasn't optimistic that senators would reach an agreement to move forward with the legislation. He did express confidence that the Senate's 41 GOP members would stick together to block a motion to proceed to debate on the bill.

"I don't believe we'll have a deal today. I believe we'll have the votes this afternoon," Shelby said during an appearance on ABC when asked about this evening's vote on Wall Street reform. 

If all 41 senators would vote against a motion to move forward with the legislation, it has the effect of blocking the bill for now.

Senate Majority Leader Harry ReidHarry ReidThis week: Congress awaits Comey testimony Will Republicans grow a spine and restore democracy? Racial representation: A solution to inequality in the People’s House MORE (D-Nev.) has scheduled a vote to begin debate for after 5 p.m. Monday, a vote scheduled Thursday after it had appeared it would take more time to negotiate a bipartisan deal on the legislation.

Shelby said that while he and Banking Committee Chairman Chris Dodd's (D-Conn.) staffs had worked throughout the weekend, a deal was still off on the horizon.

The unified GOP opposition to the bill in its current form, though, could be scuttled if even one member breaks with the party to join Democrats in advancing the bill. Sen. Bob CorkerBob CorkerClinton administration official knocks 'soap opera' of Trump White House Trump's steps on Iran show cooperation with Congress is possible Senate GOP short on ideas for stabilizing ObamaCare markets MORE (R-Tenn.) and Sen. Olympia Snowe (R-Maine), who backed strong regulation of derivatives markets in the bill, are thought to be the most likely defectors.

"What we're trying to do here is strengthen the bill, not weaken it," Shelby said of Republicans' involvement in the legislation, rejecting the suggestion that opposing the bill could be politically dangerous.

"What most Republicans and most Democrats are interested in is a strong substantive bill, and few loopholes," he said.