A clip from new documentary, (Astro) Turf Wars: How Corporate America Faked a Grassroots Revolution, ties oil billionaire David Koch closely to the Tea Party movement <a href="http://astroturfwars.com/">astroturfwars.com</a>

It likes to present itself as a grassroots insurgency made up of hundreds of local groups intent on toppling the Washington elite.

But the Tea Party movement, which is threatening to cause an upset in next month's midterm elections, would not be where it is today without the backing of that most traditional of US political supporters – Big Oil.

The billionaire brothers who own Koch Industries, a private company with 70,000 employees and annual revenues of $100bn (£62bn), used to joke that they controlled the biggest company nobody had ever heard of.

Not any more. After decades during which their fortune grew exponentially and they channelled millions of dollars to rightwing causes, Charles and David Koch are finally getting noticed for their part in the extraordinary growth of the Tea Party movement.

The two, 74-year-old Charles and David, 70, have invested widely in the outcome of the 2 November elections.

One Koch subsidiary has pumped $1m into the campaign to repeal California's global warming law, according to state records.

The brothers, their wives and employees have also given directly to Republican candidates for Congress and are the sixth-largest donors to the Senate campaign of Tea Party favourite Marco Rubio.

They have also given heavily to the Republican Jim DeMint in South Carolina, according to the Centre for Responsive Politics.

But organisations tracking money in politics say the Kochs' biggest impact in the midterm elections will be from funding and providing logistical support to such groups as Americans for Prosperity (AFP), one of the biggest Tea Party groups.

AFP, in turn, has spun off other organisations such as November is Coming, Hands Off My Healthcare, and the Institute of Liberty, which are buying up television ads and holding rallies across the country in an attempt to defeat Democrats.

US campaign laws make it easy for political interest groups and their corporate backers to hide their spending in elections. "This is a world of shadows," said Taki Oldham, an Australian documentary maker who spent months following Tea Party activists. "In my mind, without a doubt nobody has had more influence on the anti-Obama campaign than the Koch-funded groups."

For the Kochs, who inherited their politics as well as their business from their father, Fred, this has been a long and carefully cultivated project.

But after years in which their support for anti-regulation thinktanks and groups went largely undetected, the sudden visibility of the Kochs' power seems to have taken even the brothers by surprise.

"Five years ago, my brothers Charles and I provided the funds to start Americans for Prosperity," David Koch told AFP's annual Defending the Dream gathering in 2009. "It is beyond my wildest dreams that AFP has grown into this enormous organisation. The American dream of free enterprise and capitalism is alive and well."

Until last summer, most Americans had no idea who the Koch brothers were, and it is very likely that even AFP members did not know they were bankrolled by one of the richest men in corporate America.

But a spate of attention – sparked by a Greenpeace investigation and a profile in the New Yorker – has given the Kochs a degree of notoriety they are finding it difficult to live with.

When the Guardian stopped into the offices of the Charles G Koch charitable foundation, in the suburbs of Washington DC, the receptionist sent the head of the legal department out to talk.

After declaring all conversations off the record, lawyer Brian Menkes said it was normal for the Koch legal team to be involved in routine press inquiries.

"Go to any company in the world and the legal department is involved in media inquiries," he said. "It is standard operating procedure."

For his part, David Koch now seeks to distance himself from the Tea Party. In a rare interview in New York magazine, he said: "I've never been to a Tea Party event. No one representing the Tea Party has ever even approached me."

The extreme sensitivity carries across to the company website, where recent additions tout the brothers' commitment to environmental protections and offer a selection of "Koch facts", an antidote to the unflattering personal and political portraits of the two that have appeared recently.

"This is the outing of the Koch brothers. They didn't want this story told, especially in an election year," said Kert Davies, the research director for Greenpeace who has spent a decade gathering data on the family. "They have never been face forward."

But they do have deep pockets. Koch Industries has expanded from oil refinery to paper towels and Lycra.

The two brothers each own 42% of the company and occupy top-10 positions in the Forbes annual ranking of wealthy Americans, with personal fortunes of $21bn each.

Over the last 20 years, Koch Industries has donated at least $5.9m to political candidates, some 83% of which was set aside for Republican candidates, according to the Centre for Responsive Politics.

Since 1989, Koch Industries has spent more than any other oil and gas company on finding favour in Congress, paying $50m to lobbying firms.

But it is the Kochs' links to a welter of mass mobilisation campaigns opposing Barack Obama that is making the biggest impact. Political monitoring organisations say the Koch-connected Claude R Lambe Charitable Foundation has given $3.1m to Americans for Prosperity.

The Kochs' involvement in anti-government causes goes back to their father, who was a founding member of the virulently anti-communist John Birch Society.

He made his fortune by developing a more efficient refining method and built plants around the world, including 15 in the Soviet Union at the height of Stalin's purges in the early 1930s. He came to despise Stalin and, David Koch told the New Yorker, was "paranoid about communism, let's put it that way".

He worked hard to instil the same beliefs in his four sons. Two of those sons later became estranged from the family, and were eventually bought out of Koch Industries, but for Charles and David, the rightwing free market ideology was their lodestar.

As the CEO of Koch Industries, Charles Koch began supporting free-market thinkers while honing his own ideas in a book called the Science of Liberty.

The brothers did try direct action. In 1980, David Koch ran as a vice-presidential candidate on the libertarian ticket, winning just 1% of the vote.

The episode, Charles Koch has written, persuaded the brothers to refocus their energies. In the 1980s, they founded Citzens for a Sound Economy.

Over the next 20 years, they funnelled around $13m to Citizens for a Sound Economy. In 2004, the organisation split into Americans for Prosperity and FreedomWorks, which is closely tied to the former Republican Congressman Dick Armey and has received no more funding from the Koch family.

Obama's election, and the prospect that the new president would reverse nearly two decades of reduced government oversight of industry, put the Kochs and their footsoldiers in Americans for Prosperity on high alert.

"They were very afraid of the Obama administration and a return to a pro-regulatory environment after the Bush years, and probably ramped it up a bit to make sure nothing new was going to inhibit their business," Davies said.

A day after CNBC's Rick Santelli launched his on-air howl against Obama's mortgage bailout plan, AFP and Freedom Works put up Facebook pages and began organising events around the country. The Tea Party was under way.

• This article was amended on Thursday 21 October 2010. We said that there was no sign of David Koch at the Defending the Dream convention this year, but he did in fact attend the gathering, where he presented an award. This has been corrected.