Wall Street has fiercely resisted President Obama's bank reforms
US Republican senators have blocked moves to start debating a bill to introduce the most significant reforms to financial regulations for 60 years.
Needing 60 votes in the 100-seat Senate to begin debating the bill, Democratic leaders fell three votes short.
Democratic Senator Ben Nelson joined 39 Republicans in voting against it.
Republicans say the bill does not go far enough in its reforms, while Democrats say their opponents want to protect wealthy corporate interests.
Both parties say they expect the overhaul will eventually be approved.
Correspondents say that with Wall Street reeling from a fraud case against Goldman Sachs they are eager to act before November's Congressional elections.
Goldman executives are due to appear before a Senate committee later. On Monday, the committee said the bank had made billions of dollars at its clients' expense during the housing market collapse.
Minutes after Monday's procedural vote, President Barack Obama said he was "deeply disappointed" that Republicans had rejected a debate and urged senators to put the interests of country ahead of party.
"Some of these senators may believe that this obstruction is a good political strategy, and others may see delay as an opportunity to take this debate behind closed doors, where financial industry lobbyists can water down reform or kill it altogether," he said.
"But the American people can't afford that."
It is likely that the Democrats will try again later this week to push through a debate. Senate Majority Leader Harry Reid switched his vote to "no" at the end - a manoeuvre that will enable him to call for a new vote as early as Tuesday.
"The only thing Republicans stand for is standing together," he said.
The Democratic dissenter, Sen Nelson, said his vote reflected concerns about the bill from businessmen in his home state.
Nebraska-based billionaire Warren Buffett has reportedly been lobbying to exempt existing derivatives from new regulations.
Analysts say that with neither side appearing to be willing to compromise over the shape of the bill, further delays look likely.
Last-minute talks had already taken place to try to reach a compromise before the vote.
But Senate Minority leader Mitch McConnell had urged his side to stand firm.
"All of us want to deliver a reform that will tighten the screws on Wall Street. But we're not going to be rushed on another massive bill based on the assurances of our friends on the other side," he said.
Democrats and their two independent allies control 59 Senate seats, but had needed at least one Republican vote to overcome delaying tactics.
The bill's supporters say it would tackle financial institutions that are "too big to fail", putting in place a framework that would mean taxpayers do not fund any future bailouts.
Among the bill's proposals are changes to the derivatives market and tougher legislation to protect consumers.
Mr Obama has made reining in Wall Street a cornerstone of his presidency.
In a speech last week to the financial community, he attacked the army of lobbyists in Washington employed by banks to oppose his changes.
A poll published in the Washington Post on Monday suggests two-thirds of Americans support stricter financial regulations.