An oil company headed by conservative billionaires David and Charles Koch has contributed $1 million to the campaign to suspend the state’s landmark climate change law.

Flint Hills Resources does not have any oil interests in California but is a big opponent of climate change legislation around the country.

On Thursday, the Kansas-based refining and chemicals manufacturer threw its weight behind Proposition 23, the ballot initiative that seeks to suspend California’s greenhouse gas reduction law until the economy improves.

“This is a significant game changer,” said Craig Holman, a campaign finance expert at Public Citizen, a Washington, D.C.-based consumer advocacy group. “They want to stop the state that’s well known for leading the way when it comes to climate change legislation.”

California secretary of state’s office records show that Flint Hills is now the third largest backer of Proposition 23, behind Texas-based oil companies Valero Energy Corp. and Tesoro Corp.

On Thursday, Tesoro also donated $1 million to the Yes on 23 Committee, bringing its total contributions to about $1.5 million. Overall, the committee has raised more than $8.2 million, with nearly half, or about $4 million, coming from Valero.

Flint Hills spokeswoman Katie Stavinoha said the company believes that California’s law sets a bad precedent for the rest of the nation.

“Flint Hills believes that implementing the law will cause significant job losses and higher energy costs,” Stavinoha said.

Anita Mangels, spokeswoman for the Yes on 23 Committee, said Koch Industries shares many of the job-preservation goals of her group’s 300 members.

Established in 1940, Flint Hills, a wholly owned subsidiary of Koch Industries Inc., operates oil refineries in Texas, Minnesota and Alaska, as well as chemical manufacturing plants in Michigan, Illinois and Texas.

Koch Industries is one of the nation’s largest privately held companies. Its holdings include Brawny and Dixie paper products and the Lycra fiber and Stainmaster carpet brands.

According to Forbes magazine, brothers David and Charles Koch are tied as the 24th richest men in world, each with a net worth of about $17.5 billion. Among beneficiaries of their bankroll is the Tea Party movement.

Signed by Gov. Arnold Schwarzenegger in 2006, the state’s climate change law, or Assembly Bill 32, aims to reduce carbon emissions statewide to 1990’s level by the end of the decade.

Proposition 23 seeks to suspend the law until the statewide unemployment rate hits 5.5 percent for four quarters in a row.

Proponents of the climate change law say that Koch and other oil companies are trying to preserve their profits at the expense of the growing clean-tech industry and health concerns of California residents.

“What Proposition 23 is trying to do is to move our state backward,” said Tom Steyer, co-chairman of the No on 23 Committee.

With a little more than two months before the elections, the battle over Proposition 23 is shaping up to be one of the most expensive ballot initiatives this year.

Supporters of the climate change law — which include environmentalists, clean-tech advocates and private investors — have pledged nearly $10 million to defeat Proposition 23.

Steyer, a San Francisco hedge fund operator, has pledged $5 million. Other notable donors include:

— Julie Packard, philanthropist and daughter of Hewlett-Packard Co. cofounder David Packard, who contributed $101,894 on Aug. 27.

— San Francisco private equity investor Warren Hellman, who donated $75,000 on Aug. 16.

— Silicon Valley venture capitalist John Doerr, who donated $500,000 on Aug. 6.

— Wendy Schmidt, philanthropist and wife of Google Inc. co-founder Eric Schmidt, who gave $500,000 on June 16.