SACRAMENTO (AP) — Oil billionaires David and Charles Koch have jumped on board an effort to suspend California’s global warming law by making a million-dollar contribution this week.

A subsidiary of Wichita, Kan.-based Koch Industries, the nation’s second-largest private company with oil refineries and pipelines, made a $1 million contribution Thursday to the campaign for Proposition 23. It joins two Texas-based companies, Valero Energy Corp. and Tesoro Corp., in backing the measure.

According to the Los Angeles Times, a spokeswoman for Flint Hills Resources said the company “may consider additional support.”

California’s global warming law, known as AB32, seeks to reduce greenhouse gas emissions statewide to 1990 levels over the next decade.

Proposition 23 seeks to suspend California’s 2006 law until the state’s unemployment rate falls below 5.5 percent for four consecutive quarters. That level has happened only three times in the past three decades, according to state statistics.

The initiative is opposed by environmental groups, Democratic lawmakers and Republican Gov. Arnold Schwarzenegger, who called the contribution “extremely disappointing.”

Katie Stavinhoa, a Koch spokeswoman, said the company believes the law will cause “significant job losses and higher energy costs.”

The Koch brothers have founded several libertarian organizations and think tanks dedicated in large part to fighting what they view as excessive government regulation. They have helped finance efforts to develop arguments against global warming.

So far, the Proposition 23 campaign has raised $8.2 million, of which 97 percent has come from oil interests. The campaign against the measure has raised $6.6 million with donations from billionaire hedge fund manager Thomas Steyer, environmental groups and cleantech businesses.