The gasoline tax, a subject of serious debate among economists, has been resoundingly defeated every time it has come to a vote in Congress.

So why is Jimmy Carter wheeling out a gas tax?

Gasoline, after all, accounts fox only 12 to 15 per cent of total U.S. energy consumption. To reduce gasoline use, even marginally, many economists think it would have to be taxed up above the $1 a gallon Europeans pay. And such a tax, many of them argue, is severely regressive - hurting low-income families much worse than it does the wealthy.

Why then is Carter ready to risk a fight over the gas tax? For psychological reasons, his advises say.

"The American people," says energy adviser James R. Schlesinger, "have never believed in physical shortages." The gas tax is designed to make them believers.

"This is a way of making real the future problem," says White House press secretary Jody Powell. He calls it a "psychological weapon," aimed at giving every driver a stake in curbing excessive use of gasoline.

The Carter tax would go into effect only when energy use exceeded specific targets. Powell says motorists will understandthat "if we hold consumption down, we don't suffer; if we don't hold consumption down, we do suffer. You slap your own wrist."

The "slap on the wrist" could be the focal point of Carter's energy battle, with many of the congressional heavies weighing in against it.

Sen. Henry M. Jackson (D-Wash.) says, "In my judgement, a gas tax won't get you anywhere."

Rep. Thomas L. Ashley (D-Ohio) points out, "Taxes on gasoline are an anathema . . . The House has voted accordingly in the past, but I'm not sure we couldn't get it through."

Patrick Caddell, the President's pollster, has told the White House that more than 60 per cent of the American people oppose raising gas taxes.

If that same percentage holds in Congress on Carter's standby tax, it could be more than a symbolic setback for the President.