Empty gesture, for so many reasons.
First, if you use 15 gallons of gas a week, eliminating the federal tax from Memorial Day to Labor Day is going to save you a whopping $38 or so, or a little less than 3 bucks a week.
Second, to the extent that high energy costs are related to the falling value of the dollar, this proposal does absolutely nothing to address those costs. Crude is up somewhere near $112/barrel. The dollar is down to .632 euros. Meaning oil is still 70 euros a barrel. High by historic standards, certainly, but not rising as fast for people buying with euros as for people buying with dollars.
Third, to the extent that market forces are responsible for high fuel costs, eliminating the tax will further stimulate demand for oil, raising prices even higher. I thought Republicans didn't like to fuck with the market?
We had an opportunity, back in the mid-late 90s, when oil was $12/barrel or so and gas was about a buck twenty a gallon, to slap a pretty high tax on fuel and use that revenue to both repair our infrastructure and fund research and development on alternative fuels. Instead, we chose to "let the market" decide. The result was a generation of gas guzzlers and the only "alternative" the market created was ethanol. And we can see how well that's working out.
Really, if this is McCain's best, it's not hard to imagine America as a full fledged Third World country at the end of a McCain presidency.