The financial storm that has been battering Wall Street has also blown away John McCain's lead in the presidential race, where a five-point lead in some polls has turned into a five-point deficit within three days.

With 46 days to go until America votes, opinion polls are volatile, but the shift in momentum to the Obama campaign has underlined his rival's apparent vulnerability on the economy.

A clutch of new polls showed a lead for the Democrat, with The New York Times/CBS News poll giving him a 48-43 margin over the Republican Mr McCain. The gap is within the margin of error in all of the polls, but represents a significant shift.

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Barack Obama was intent on pressing home the economic attack. He promised to bring change by challenging the "old boys network" in Washington. "In the McCain campaign that's called a staff meeting," he quipped.

But Senator McCain was on the offensive at an enthusiastic rally in Cedar Rapids, Iowa. He blamed regulators who were "asleep at the switch" for the meltdown of the financial system, rather than the banks who had made the bad housing loans. They had "turned the financial markets into a casino," he said.

To cheers, he declared that he would fire the head of the Securities and Exchange Commission, which regulates Wall Street.

The McCain camp's attempt to distance itself from the economic legacy of President George Bush, a fellow Republican, is having limited effect so far. The latest polls reveal that despite his efforts, voters believe he is far less likely to make changes than his adversary. The Arizona senator is still seen as a "typical Republican" who would entrench current unpopular policies, rather than get the country back on track.

The daily crop of grim news from Wall Street has sent shockwaves across America, where voters live in dread of losing their jobs as the fallout triggers a tide of company closures and job losses.

The latest polls also show that the "Palin effect" – the bounce the McCain campaign enjoyed after the nomination of Alaska's governor, Sarah Palin, as the vice-presidential candidate – has run its course. There was an initial surge of interest in the McCain-Palin ticket, especially among white women voters, which energised the evangelical conservative base of the Republican Party.

But scrutiny of Governor Palin's record on issues such as abortion and her inexperience on the national stage appears to have rallied the so-called "Clinton Democrats" to the Illinois Senator's side. Some observers believe she may soon become a liability to the Republicans. Her "favourability score" averaged across four recent polls gives her the worst scores among the four candidates for the White House and vice-presidency.

At the heart of the intensifying race for the White House is a tug-of-war between the two campaigns over independent voters, who are notoriously fickle. After switching to the McCain ticket following the Republican convention they are now roughly split, according to a different Quinnipiac university poll, which put them at 46 per cent for Mr Obama and 45 per cent for Mr McCain yesterday. The New York Times gives Mr Obama a major lead among voters aged 18 to 44, while his rival leads by 17 points among white men and by the same margin among voters aged 65 and over. Middle-age voters – 45 to 64 – are evenly split between the two.

The Obama campaign hopes it can stay focused on the economy. But the first presidential debate in Mississippi next week will focus on foreign policy, where Mr McCain is generally seen as stronger. Surveys give him a substantial advantage over his rival as the potential commander-in-chief. And for the first time, a majority of Americans say that the "surge" of US troops in Iraq, which Mr McCain advocated, has made a big difference there.