LOS ANGELES  A ballot initiative to suspend a milestone California law curbing greenhouse gas emissions is drawing a wave of contributions from out-of-state oil companies, raising concerns among conservationists as it emerges as a test of public support for potentially costly environmental measures during tough economic times.

Charles and David Koch, the billionaires from Kansas who have played a prominent role in financing the Tea Party movement, donated $1 million to the campaign to suspend the Global Warming Solutions Act, which was passed four years ago, and signaled that they were prepared to invest more in the cause. With their contribution, proponents of the proposition have raised $8.2 million, with $7.9 million coming from energy companies, most of them out of state.

This latest embrace by the Koch brothers of a conservative cause jolted environmental leaders who are worried that a vote against the law in this state  with its long history of environmental activism  would amount to a powerful setback for emission control efforts in Washington and statehouses across the country.

“It would have big implications,” said George P. Shultz, the former secretary of state, who is a chairman of a campaign to defeat the ballot initiative. “That is one reason why these outside companies are pouring money in to try to derail the same thing. At the same time, the reverse is true: they put this fat in the fire and if we win, that also sends a message.”