as few as 20 permanent new jobs in the U.S., and the crude delivered by Keystone XL will be exported into the global market from U.S. ports.

Because the pipeline would cross the border between the United States and Canada, it must get a permit from the State Department after environmental and other reviews. President Obama has said he will make the decision about whether to approve the pipeline based in part on whether those reviews conclude that the pipeline contributes significantly to carbon pollution.

In the new television ad, Mr. Steyer claims that has already been answered.

“The truth? Scientists report Keystone oil would produce as much carbon pollution as 620,000 cars every year,” he says in the ad.