Suppose that Clinton and McCain are able to get a gas tax holiday passed for this summer. This despite the fact that
1. Clinton can't name a single economist who thinks the tax holiday is  a good idea,
2. Pelosi said it wouldn't come up this spring in the House,
3. senators are not stupid and don't like being challenged, especially Democratic ones who are superdelegates, and
4. no one trusts Congress -- not even Congress -- to tax excess oil profits and George Bush would veto an excess profits bill even if Congress did manage to pass the tax.
Anyway, we get a tax holiday. Two things can occur: the tax holiday works to reduce gas prices or the tax holiday fails to make any noticeable difference in gas prices. Again, let's take the optimistic choice, optimistic for McCain and Clinton, that gas prices drop.
The people who allowed themselves to be pandered to, the voting public, enjoy somewhat cheaper gas all summer. Come Labor Day, the tax holiday expires. Now prices will automatically rise 18 cents a gallon.
The panderers come out of the woodwork. They cry foul: a tax increase -- on working Americans! Congress gets intimidated. The rush is on. Interns and staffers get run over as Congress votes to make the holiday permanent -- perhaps until such time as gas prices drop.
There's no offsetting tax on excess profits. There never has been in the last sixty or so years. Dedicated funds for the infrastructure are lost forever. Congress has to allocate specific highway and bridge funds. Highways in Alaska and Nevada get much better while the infrastructure crumbles in states like Indiana and North Carolina with less influence.
Soon pressure builds up in the states for their own gas tax holidays. Things get worse. The pandered public thinks it's OK because their president thought so. Real American jobs that can't be outsourced are lost.
Meanwhile, back in the Oval Office, the new president congratulates her- or himself on a wise political move.