Senate Majority Leader Harry Reid (D-Nev.) vowed Monday to move forward with a financial regulatory overhaul this week, despite last-minute waffling by Sen. Ben Nelson (D-Neb.) that could throw that schedule into doubt.

“We will finish our work on this bill this week to ensure that these critical protections and accountability for Wall Street are in place as soon as possible,” Reid’s office announced Monday night.

But to do so — without waiting for West Virginia Gov. Joe Manchin (D) to name a successor to the late Sen. Robert Byrd (D) — Reid needs the support of Nelson, who is suddenly showing signs of wavering.

Nelson refused on Monday night to commit to voting for the bill and listed a number of concerns, including criticism of its consumer protection agency provisions, according to the politics blog Talking Points Memo. “You don’t know who’s going to be head of the consumer protection bureau. … You can’t just send a rogue agency out on its own,” Talking Points Memo quoted Nelson as saying.

Over the last several weeks, Reid and other leaders have operated under the assumption that Nelson would back the financial regulatory overhaul. Under that calculus, Reid would have the support of 57 Democrats — Sen. Russ Feingold (D-Wis.) is firmly opposed, and West Virginia is down a vote because of Byrd’s death.

As a result, Democrats needed the votes of three Republicans, and Reid and Banking Chairman Chris Dodd (D-Conn.) focused their efforts on wooing GOP moderates Susan Collins (Maine), Olympia Snowe (Maine) and Scott Brown (Mass.).

Collins indicated last week that she would back the bill, and Reid seemed confident he could muster enough GOP support to move the bill this week. But a few hours after Brown announced he was backing the bill and as Snowe was making the same decision Monday, Nelson indicated his vote was not as certain as Reid may have thought.

Although leadership aides said Democrats are confident they can persuade Nelson to vote for the measure and deliver on Reid’s vow to finish the bill this week, Nelson has become increasingly unreliable for Democrats.

After taking a political beating from Republicans following the notorious “Cornhusker Kickback” provisions in the health care overhaul, Nelson has become increasingly hostile to the Democratic agenda. He has largely voted in lock step with Republicans on a host of jobs and economic issues that did not include offsets.

Nelson’s office did not respond to repeated requests for comment.