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 Mason ReidMcConnell not yet ready to change rules for Trump nominees The Hill's Morning Report — Sponsored by CVS Health — Trump’s love-hate relationship with the Senate Trump to press GOP on changing Senate rules 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 CorkerRobert (Bob) Phillips CorkerAdministration works to assuage critics over ZTE deal Overnight Defense: Trump decision on Korea summit coming 'next week' | China disinvited from major naval exercise | Senate sends VA reform bill to Trump Senate sends major VA reform bill to Trump's desk 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.