Romney, Obama spar over impact of mileage boost

One of Republican presidential nominee Mitt Romney's top campaign pitches is his plan to achieve North American energy independence by 2020 through expanded domestic oil and gas exploration and reliance on imports from Canada and Mexico.

But that's a claim President Barack Obama's campaign contends can only be met if Romney relies on the higher auto mileage standards Obama has put into place -- the same ones that Romney opposes.