Al Jazeera's John Terrett, reporting from New York, said: "Investors are not convinced that the plan is big enough, or that the proposal will be approved in time or will work."
Investors fear that the government will have to dramatically ramp up borrowing to pay for the mammoth rescue effort, an inflationary move that could further devalue the dollar and trigger another surge in commodities prices, he said.
Bernanke is due to warn the senate banking committee that, despite unprecedented steps already taken in the crisis, global financial markets "remain under extraordinary stress".
Bernanke said before the congressional address that action by the legislature was "urgently required to stabilise the situation and avert what otherwise could be very serious consequences for our financial markets and for our economy".
Wall St 'blank cheque'
But congressmen were still at odds with the treasury on several issues.
Democrats, who control both chambers of congress, said that the White House had agreed to an oversight board to monitor the bail-out, one of their key demands, but differences remain on levels of compensation for executives of rescued companies, or whether they should receive anything at all.
"We are not sending a blank cheque to Wall Street," Nancy Pelosi, the House speaker, said after holding bipartisan talks on Monday.
Paulson is expected to repeat his argument that the bail-out will prove cheaper for taxpayers than leaving companies to suffer the cost of the crisis themselves.
The scramble to steady the US financial ship has left one man with enormous power to steer its course, with some already calling Henry Paulson, the treasury secretary, King Henry.
Paulson had the same powers as his predecessors until a couple of weeks ago: oversight of the government's taxation and fiscal policies.
But as the financial storm erupted, with some institutions crumbling, sold or bailed out, Paulson's new job description has changed substantially.
If approved by congress, his rescue plan would give him sole control of a $700bn line of credit, and the power to use it to take on any bad mortgage-related debt he chooses.
Congress, and the two main presidential candidates, John McCain and Barack Obama, have called for greater oversight of such powers.
Some critics have expressed concern over the unprecedented power that the plan would give Paulson and the US government in the financial markets, and whether it would find itself saddled with a gigantic debt.
But George Bush, the US president, pushed congress for progress a day earlier, warning that "failure to act would have broad consequences far beyond Wall Street".
Dana Perino, the White House spokeswoman, said that Bush would make the case that the US has taken "aggressive, decisive action" on the situation when he meets world leaders at the UN General Assembly in New York later on Tuesday.
Traders shunned the US stock market on Monday and as investors moved their money into tangible assets such as oil and gold, causing commodities prices to surge.
Oil prices jumped more than $25 a barrel to $120.92, recording their biggest ever one-day gain before falling early on Tuesday to $107.24.
Japanese markets were closed on Tuesday for a national holiday but share prices in Hong Kong, Australia, South Korea and Singapore were all lower in early trading.
The FTSEurofirst 300 index of top European shares was down 1.8 per cent at around 09:00 GMT on Tuesday, with banks the biggest drag on the index.
London's FTSE 100 was down 0.08 per cent, Frankfurt's DAX 30 shed 0.26 per cent and the CAC 40 in Paris down 0.62 per cent.
"The bail-out plans are coming under severe pressure now and there are more concerns surfacing then questions being answered. What we're going to need is a little more clarity," James Hughes, market analyst at CMC Markets in London, said.
"Something needs to be said, which is going to calm markets down a little bit and that should start to stabilise things a little bit."