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Barack Obama kicked off a week devoted to his energy plan with the release of a negative ad going after John McCain's relationship with Big Oil.

As the American public continues to reel from $4 gasoline, both parties are trying to turn the energy issue into a political weapon. Obama announced this weekend that he would reluctantly support the expansion of offshore domestic oil drilling -- a measure McCain has been pushing -- as part of a bipartisan compromise on energy issues. Critics are labeling this as another in a series of Obama flip-flops, but the Illinois senator is presenting it as evidence that he is willing to be flexible and work with Republicans to pass a package of effective legislation on an issue that has so far stymied both houses of Congress.

But such cooperative, bipartisan sentiments are less evident in Obama's new TV spot. "Pocket" (subscription) accuses McCain of taking money from Big Oil while supporting tax cuts for oil companies that are already making record profits.

"Every time you fill your tank, the oil companies fill their pockets," the announcer charges, as images of people pumping gasoline appear on screen. "Now Big Oil's filling John McCain's campaign with $2 million in contributions," he claims. The camera zooms in on a picture of President Bush and McCain as the announcer continues: "After one president in the pocket of Big Oil, we can't afford another." Finally the announcer mentions Obama's plan to give every family a $1,000 tax rebate to defray energy costs, which he would finance by taxing oil companies' windfall profits.

Reaction from the McCain campaign was fast and furious, attempting to reinforce the message that Obama is an out-of-touch, rock-star candidate. "Barack Obama's latest negative attack ad shows his celebrity is matched only by his hypocrisy," McCain spokesman Tucker Bounds said in a statement. He claimed that Obama himself has taken nearly $400,000 in contributions from employees of oil and natural gas companies, and added that Obama voted yes on a 2005 energy bill that gave tax breaks to oil companies, a measure that McCain rejected.

Former Massachusetts Gov. Mitt Romney, a McCain surrogate and potential running mate, also responded to the ad on CNN's "American Morning." "Well, that's really sad. I didn't know that Barack Obama had stooped to dishonesty," the former presidential candidate said, noting that McCain's plan includes tax cuts for all corporations, not just oil companies.