While the House is still expected to pass the final bill soon, it was unclear when the Senate would act, or whether Democrats still had enough support to enact it.
Byrd's death early Monday came three days after Sen. Scott Brown, R-Mass., threatened to yank his support for the bill. Brown's vote is seen as vital to reach the necessary 60 votes to push the legislation to final enactment.
"Between Brown's suggestion that he may not vote for it and with Byrd's passing, now they have to scramble to get the votes," said Brian Gardner, an analyst with KBW Inc. "It may get delayed. If it doesn't get wrapped up this week, it certainly adds a level of drama and a level of uncertainty. That uncertainty can lead to some mischief."
Ultimately, most observers said, reform is still likely to be enacted. But with legislation that has already included dozens of fits and starts, the last-minute drama over votes made many uneasy.
The reform bill had little margin for error even before Byrd's death.
Though four Republicans, including Brown, supported the legislation during Senate debate, two Democrats opposed it — Sens. Maria Cantwell and Russ Feingold.
Without Byrd, whose death knocked Democrats down to 58 votes, Democrats need to hold all Republican votes or pick up one or both defectors to get the necessary 60 votes.
For his part, Brown cast doubt on his support in a statement issued Friday, saying that a provision added at the eleventh hour to subject banks to a tax to pay for costs from the bill was troublesome.
"I was surprised and extremely disappointed to hear that $18 billion in new assessments and fees were added in the wee hours of the morning by the conference committee. My fear is that these costs would be passed on to consumers," Brown said. "I've said repeatedly that I cannot support any bill that raises taxes."
Chris Low, a chief economist with First Horizon National Corp.'s FTN Financial, said Democrats "need Scott Brown."
Alternatively, he said they will need to pressure Feingold and Cantwell to switch sides. The problem, Low said, is that leadership cannot offer much in return for their votes, at least not on financial reform.
Under the conference rules, the legislation cannot be changed on the floor in either chamber, leaving lawmakers with a straight up or down vote on the bill.
"At this point you can't really offer anyone anything," Low said. "And there's a real sense in Congress that they are running out of time to finish up the things they want to do for the year. They only have seven legislative weeks left."
Further complicating the situation is whether the other three Republican supporters — Sens. Susan Collins and Olympia Snowe of Maine and Sen. Chuck Grassley of Iowa — could join Brown in turning against the bill. Republicans historically are opposed to any increase in taxes.
"I think it's a lot more serious than people think," said an industry lobbyist, who spoke on condition of anonymity. "Any time you raise taxes, Republicans typically have an ideological preference against it. There's a possibility you lose all four Republicans. The outcome of the bill hangs in the balance."
Spokespeople for Grassley, Brown, Snowe and Collins would not say how their bosses would vote on the regulatory reform bill Monday, and a spokesman for Cantwell said that they were still reviewing the legislation. Feingold released a statement Monday that said he would not support the bill.
