Federal Gas Tax Holiday

How about that Federal Gas Tax holiday that Clinton and McCain are recommending for this summer? My conservative friends keep reminding me that the price of gasoline is dependent on supply and demand. The capitalist system is working just fine. Decreased supply of a product leads to increased price. So what happens if gasoline is made cheaper by a Federal Gas Tax holiday. People will buy more gasoline and the price will continue to go up. Oil companies make even more profit and badly needed Federal tax revenues go down. Or am I missing something?