I respectfully, but strongly, disagree.
The more gas prices go down, the better McCain's chances are of winning the election because people will be happier about the economy and less angry with the president.
And they will be right to be happy because lower gas prices will improve the economy.
Believe it or not, ignorant voters (most voters) who don't know how things work, blame the president and Republicans for higher gas prices.
I think you would be surprised to learn the percentage of people who still don't know that Democrats actually have controlled Congress since 2006.
McCain's best (maybe only) chance of winning in 2008 is if the economy and George W. Bush's approval rating improve. Lower gas prices would improve both.