When Gov. Arnold Schwarzenegger signed California's landmark law to curb greenhouse gas emissions in 2006, he was unequivocal about its importance. "There is nothing that is more important than protecting our planet," the governor intoned.

"So I hope that all of you are as proud as I am today," he added, "as California leads the way in one of the most important issues that are facing our time, which is the fight against global warming and protecting our environment."

Schwarzenegger's signing of Assembly Bill 32 was momentous, but the real test of his legacy is whether the measure effectively reduces emissions that are warming the plant. To do that, California will need to rapidly improve energy efficiency, change its sources of electricity, promote alternative transportation and take other steps to reduce the pollutants that waft from automobiles and other sources.

Toward that goal, California lawmakers this year have sent Schwarzenegger another landmark bill, Senate Bill 375, that will go a long way toward improving regional planning, promoting transit and preventing the growth of long-distance commutes.

This bill is the work of Sen. Darrell Steinberg, the incoming Senate leader from Sacramento. After two years of painstaking negotiations and bill revisions, Steinberg has managed to bring together an unlikely coalition of environmentalists, the building industry and cities around a bill that has garnered national attention. It is now on the governor's desk.

Despite that achievement, there were reports Thursday from the governor's office that Schwarzenegger may veto SB 375. If that were to occur, it would undermine the governor's hard-earned environmental legacy, and even worse, sour his dealings with the incoming Senate president pro tempore.

Why would Schwarzenegger play this card? We don't know. His office declined comment. We'd like to think that, if a veto is being considered, the governor has real policy concerns with the bill.

One aspect of SB 375, favored by the building industry, is that it grants relief from the California Environmental Quality Act for developments that comply with regional strategies for constraining leap-frog growth. During negotiations, some Southern California transportation agencies wanted road projects also exempted from CEQA. Since they lost that bid, it's possible they are pressuring the governor to veto the bill.

But there's another possibility, and it is far more disturbing. Schwarzenegger has been upset that Democrats didn't pass a multibillion-dollar bond measure this session to fund reservoirs and other water projects. It could well be that the governor wants some promises from Steinberg on a water bond before he agrees to sign SB 375.

If that's the case, the governor might want to think long and hard about the consequences.

Steinberg has made clear that, in his first months of leadership, he wants to work on health care reform, political reform and water – all priorities of the governor. He has a proven record of working with diverse parties to craft deals.

But if Schwarzenegger uses strong-arm tactics in a naked attempt to extract promises from Steinberg, it could poison a budding relationship between these leaders before it has a chance to flower.

Play your cards carefully, Governor. And preserve you legacy while you're at it. Sign Senate Bill 375.