The US midterm elections are on course to become the most expensive in history next month, estimated at well over $5bn (£3.15bn) – an indication of how much is riding on the outcome of the biggest test of US public opinion since the 2008 White House race.

With the Democrats facing electoral disaster and Barack Obama battling to save his presidency, the Republicans are resurgent, their campaign chests bursting with money from big corporations whose spending power has been unleashed by a supreme court ruling earlier this year providing anonymity for donors.

The estimated $5bn dwarfs the $1bn spent on the White House race.

Public Citizen, a non-profit organisation that tracks corporate spending on elections and lobbying, said today Republicans had received six times more cash than the Democrats last month, and this could rise to 10 to one this month. Much of the cash had come from Wall Street, banking and the health and pharmaceuticals industry, it said.

"We are going to see record amounts. This is the first year in which all limits are removed. The supreme court ruling reverses a century of political tradition in the US in which corporations are not supposed to get involved," said Craig Holman, Public Citizen's representative on Capitol Hill.

At stake on 2 November are 37 of the 100 US Senate seats, all 435 House seats, 37 of the 50 governorships and a raft of state legislative places. Voting has already begun in seven states.

Polls suggest the Democrats face their biggest drubbing since 1994, losing control of the House of Representatives to the Republicans, and seeing their nine-seat majority in the Senate reduced to about two or three, with an outside chance of losing it altogether.

As an indication of the historic scale of the looming defeat, a House seat in the south that has not been Republican since 1875 could now go either way.

Larry Sabato, the director of the University of Virginia's centre for politics, is writing a book about these midterms elections. He is so confident about the outcome that he already has the title: The Pendulum Swings.

"These elections will determine how much freedom of movement President Obama has in the second half of his term. That is what it is about," Sabato said. "There are only two possibilities: he is going to have modest freedom or no freedom."

With the House in Republican hands, US politics will become even more polarised, probably dashing Obama's hopes of getting through any legislation on climate change or immigration reform, or even another economic stimulus package, let alone the money needed to close Guantánamo Bay.

The Republicans would also try to unpick his health reform legislation.

Some of the biggest individual groups channelling money to the Republicans are American Crossroads, run by George W Bush's former political strategist Karl Rove, and the pro-business advocacy organisation Americans for Job Security. Under the supreme court ruling, they do not have to disclose who is giving.

On the left, organisations such as Move On are only able to match the funding they provided for the 2006 midterm elections; and, while the trade unions have raised a bit more, they have not even remotely been able to match the big corporations.

David Levinthal, a spokesman for the Washington-based Center for Responsive Politics, which monitors spending on Congressional elections, predicted the 2012 ones would set a new record. "We fully expect that the federal elections will cost at least $3.7bn and, quite possibly, could well exceed $4bn, given the amount of activity we are seeing.". The corresponding figure in the 2006 Congressional elections was $2.8bn.

The Montana-based National Institute on Money in State Politics (NIMSP), which monitors the governors' races and state legislators, records $1.4bn in donations so far, most of it in the last six months.

That, along with the CRP figure, puts the estimate for the elections at $5.1bn , equivalent to the annual running costs of a small US city. The NIMSP records only donations made so far, and there is still a month to go, one that normally sees the heaviest level of donations and spending.

One record for a gubernatorial election has already been broken by Meg Whitman, the former eBay chief and Republican candidate in the race to replace Arnold Schwarzenegger as governor of California, who has so far spent $119m – $10m more than Michael Bloomberg in his re-election campaign for mayor of New York last year.

Change in midterm elections is almost a reflex action for Americans, who appear to dislike one party having control – as the Democrats currently do – of the White House, Senate and House. But Professor Mark Blyth, who teaches politics at Brown University, said the US system of checks and balances is a romantic, 18th century notion inappropriate to the 21st century. "It is like giving the Starship Enterprise the steering apparatus of the Titanic. You end up with gridlock," he said.

Professor Ross Baker, a political scientist at Rutgers University, sees something more momentous happening this year than just a reflex response. "These elections are potentially revolutionary, not in the sense that they will resonate a century later, but in the sense of having an impact over the next decade," said Baker.

At issue is not only the fate of the Obama presidency but the impact of the grassroots conservative Tea Party movement on the Republicans.

"It is very rare for a third party to succeed. What typically happens is that a faction takes over an established party. In 1896, the populists took over the Democrats and in 1972 the pacifist wing took over the Democrats. What we are seeing is another of these historic captures. I think that is what is happening in the Republican party," Baker said. Even if some of the Tea Party activists who produced surprises during the primary elections, such as Christine O'Donnell in Delaware, fail to make it to the Senate, polls suggest a hardcore of Tea Party-backed candidates will.

Among these are Rand Paul, in Kentucky, Marco Rubio, in Florida and Joe Miller, in Alaska. All are unlikely to co-operate with Obama, but equally they could be a headache for the Republican establishment.

The Democratic pitch in the campaign is to portray the Republicans as, in the words of the vice-president, Joe Biden, "too extreme to lead", and to cite the policy positions of candidates such as O'Donnell.

Tad Devine, a long-term Democratic strategist who is advising candidates in several races and is producing ads, is unashamed about going negative. "The ads have gone negative quickly," he said, adding: "Obama was campaigning on hope; this current election is about hope and fear. Obama is running the spectre of what will happen if the Republicans take over; our side is beginning to kick in."

With Obama's popularity down from 70% after his inauguration in January last year to under 50% now, many Democratic candidates are distancing themselves from him in their campaign literature.

Chet Edwards, a Democratic congressman from Texas who is reported to have been considered by Obama as his vice-presidential running-mate, has an ad running that could have been written for a Republican, boasting of how he had stood up to Obama and Pelosi and voted against the healthcare bill. It illustrates just what desperate days these are for the Democrats.