WASHINGTON (Reuters) - A gas tax holiday proposed by U.S. presidential hopefuls John McCain and Hillary Clinton is viewed as a bad idea by many economists and has drawn unexpected support for Clinton rival Barack Obama, who also is opposed.

"Score one for Obama," wrote Greg Mankiw, a former chairman of President George W. Bush's Council of Economic Advisers. "In light of the side effects associated with driving ... gasoline taxes should be higher than they are, not lower."


Republican McCain and Democrat Clinton, who is battling Obama for their party's nomination, both want to suspend the 18.4-cents-per-gallon federal gas tax during the peak summer driving months to ease the pain of soaring gas prices. The tax is used to fund the Highway Trust Fund that builds and maintains roads and bridges.

Economists said that since refineries cannot increase their supply of gasoline in the space of a few summer months, lower prices will just boost demand and the benefits will flow to oil companies, not consumers.

"You are just going to push up the price of gas by almost the size of the tax cut," said Eric Toder, a senior fellow at the Urban-Brookings Tax Policy Center in Washington.