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.

After decades during which their fortune grew exponentially and they channelled millions to rightwing causes, Charles and David Koch are finally getting noticed for their part in the extraordinary growth of the Tea Party movement.

Charles, 74, and David, 70, have invested widely in the outcome of the Nov 2 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 those tracking money in politics say the Kochs' biggest impact in the midterms will be from funding and providing logistical support to such groups as Americans for Prosperity (AFP), one of the biggest in the Tea Party movement.

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 TV 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 filmmaker who spent months following Tea Party activists.

For the Kochs 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. — The Guardian, London