US lawmakers have expressed strong scepticism about a bail-out of the banking system, following a five-hour Senate hearing on the rescue plan.
Treasury Secretary Henry Paulson told the banking panel that delaying the $700bn (£382bn) bail-out would put the entire US economy at risk.
Lawmakers say they want assurances that the plan will benefit ordinary American home-owners as well as Wall Street.
Some have gone further, calling the plan a potential waste of public money.
"Without question, our markets and financial institutions need serious attention," said the leading Republican on the Senate Banking Committee, Richard Shelby.
"I do not believe, however, that we can solve this crisis by spending a massive amount of money on bad securities."
Committee chairman Chris Dodd, a Democrat, called the package "unacceptable" in its present form.
'Best protection'
The White House has called on Republicans and Democrats to work together to approve the plan, under which a federal fund could buy bad debt from financial institutions with "significant operations in the US".
The fund would aim to sell off these mortgage-related debts in the future when, the Treasury says, their value might have risen.
"The best protection for the taxpayer... is to have this work"Addressing the committee, both Mr Paulson and Federal Reserve Chairman Ben Bernanke said the bail-out was vital.
"Action by the Congress is urgently required to stabilise the situation and avert what otherwise could be very serious consequences for our financial markets and for our economy," Mr Bernanke said.
Mr Paulson, meanwhile, called the proposal "the single most effective thing we can do to help home-owners, the American people and to stimulate our economy".
"The best protection for the taxpayer... is to have this work," he said.
But senators from both parties have voiced concerns that taxpayers would be paying a huge price for mistakes made by banks.
They also said it was crucial not to rush through the bail-out - which is unprecedented in US history - without carefully considering how it would work.
Democrats are seeking greater oversight of the plan, help for Americans who stand to lose their homes and limits on compensation for executives at firms selling bad assets.
The Democratic senator for New Jersey, Robert Menendez, warned against being "stampeded into bad decisions".
But he said there was a consensus that action had to be taken.
"The question is - is it the right action at the end of the day? You can take the wrong action, and that won't do anything for markets and the long-term process."
Mr Bernanke and Mr Paulson are scheduled to testify to a House of Representatives committee on Wednesday.
Global markets have been focused on the wrangling over the bail-out.
News of the plan late last week led to a rally, but this week has seen further falls amid concern that it could be delayed or watered down.
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