I’ve been on the road (actually doing a public dialog with Barney Frank on financial reform), so I’m just catching up. Anyway, John McCain has a really bad idea on gasoline, Hillary Clinton is emulating him (but with a twist that makes her plan pointless rather than evil), and Barack Obama, to his credit, says no.

Why doesn’t cutting the gas tax this summer make sense? It’s Econ 101 tax incidence theory: if the supply of a good is more or less unresponsive to the price, the price to consumers will always rise until the quantity demanded falls to match the quantity supplied. Cut taxes, and all that happens is that the pretax price rises by the same amount. The McCain gas tax plan is a giveaway to oil companies, disguised as a gift to consumers.

Is the supply of gasoline really fixed? For this coming summer, it is. Refineries normally run flat out in the summer, the season of peak driving. Any elasticity in the supply comes earlier in the year, when refiners decide how much to put in inventories. The McCain/Clinton gas tax proposal comes too late for that. So it’s Econ 101: the tax cut really goes to the oil companies.

The Clinton twist is that she proposes paying for the revenue loss with an excess profits tax on oil companies. In one pocket, out the other. So it’s pointless, not evil. But it is pointless, and disappointing.

Add: Just to be clear: I don’t regard this as a major issue. It’s a one-time thing, not a matter of principle, especially because everyone knows the gas-tax holiday isn’t actually going to happen. Health care reform, on the other hand, could happen, and is very much a long-term issue — so poisoning the well by in effect running against universality, as Obama has, is a much more serious breach.