As Dean Baker, pointed out, McCain's proposal for a summer "gas tax holiday" is just about as bad as policy can get. Since oil companies are already producing at full capacity, supply is essentially fixed, meaning that we're at the near-vertical portion of the supply curve, and the price is determined by demand, not supply. As a consequence, temporarily eliminating the gas tax will just just shift the vertical curve downward, resulting in no change at all in the price consumers face. I've drawn the relevant supply and demand curves below. The only effect of eliminating the tax is that the producer surplus increases, i.e. the value of the tax is now captured by oil companies rather than the government.
Now, lo and behold, Clinton has embraced the same pandering proposal. Obama responded correctly:
It was bad enough to see Clinton and McCain double teaming Obama with bogus attacks. It's even worse to see Clinton latch on to McCain's bad economics.
Obama spoke out against halting a tax on gasoline during the summer months, a move supported by Clinton and presumptive Republican nominee John McCain, saying it may not bring down prices and would deplete a fund used for building highways.
UPDATE: There's a post by Poblano at Daily Kos with more detail on this.