NEW YORK (AP) — Sure, a gas-tax holiday seems like a good idea, at first. According to the presidential candidates touting it, it would yield cheaper gas.

It’s trickle down at its best ... until you drive over one of those potholes that rattles your teeth.

That’s because the idea proposed by presidential candidates John McCain and Hillary Clinton to ease the federal gas tax would strip the government of about $10 billion for transportation projects.

And the plan doesn’t even guarantee that lower taxes will lead to lower gas prices. Just the opposite might happen if the tax holiday spurs more demand for gas at a time when supply is already constricted. It’s basic economics. But it gets muddled when mixed with politics.

McCain and Clinton are trying to capitalize on motorists’ dismay over prices topping $3.60 a gallon nationally. That’s more than 20 percent higher than a year ago and it’s hurting Americans as they face mounting economic stress due to soaring food costs, fewer jobs and declining home prices.

McCain, the likely Republican nominee, recommended the gas-tax holiday during an April 15 speech. He called for cuts in the federal gas tax — 18.4 cents a gallon for gas and 24.4 cents for diesel — from Memorial Day to Labor Day — the peak summer driving season. He also wants the United States to stop adding its emergency stockpile, a move that would lower the worldwide demand for oil.

Combined, he said, the two proposals would reduce gas prices, which would have a trickle-down effect. “Because the cost of gas affects the price of food, packaging, and just about everything else, these immediate steps will help to spread relief across the American economy,” McCain said.

Clinton, who is still battling with Barack Obama for the Democratic Party nomination, announced her support of the tax holiday on April 28, and said she would make up for that lost revenue by imposing a windfall profit tax on oil companies. “I understand the American people need some relief,” she said.

Before anyone starts counting on the potential savings, it’s important to know that a tax moratorium might not give them much.

As economists explain it, gas prices rise in the summer because demand is higher. Now, refineries are already running near capacity. If lifting the tax reduces gas prices, that could boost demand even more. Supply can’t grow so it could drive prices back up.

That means the economy wouldn’t get the intended stimulus, and consumers would see no benefit or could even be worse off.

“If the supply is really fixed, then no cut in the federal gas tax will change gas prices,” said Jerry Taylor, senior fellow at the libertarian Cato Institute.

Meanwhile, the oil and gas companies — which already have been tallying massive profits thanks to higher gas prices — could see another boon to their bottom line if prices rise and they don’t have to deduct any tax from their prices, notes Len Burman, director of the nonpartisan Tax Policy Center.

“As if Exxon Mobil doesn’t have enough,” Burman said, referring to the oil giant that reported first-quarter earnings of $10.9 billion.

There is also more to this issue than just the quick fix for the economy. As noted above, it would strip the government of funds for transportation needs according to the Tax Policy Center, a tax issues think tank. Besides, this effort would encourage more consumption when the world is trying to lower it to reduce global warming.

Obama’s rivals have attacked him for not supporting the gas-tax relief — but he might have some insight into this issue that the others do not. Back in 2000 when Obama was in the Illinois Senate, he supported suspending gas taxes in his state, a moratorium that didn’t turn out to be much of an economic boon at all.

A study by the National Bureau of Economic Research of the short-term tax breaks in Illinois, and Indiana during the same period, found that gas prices — which were around $2 a gallon then — fell by an average of 3 percent, not the full 5 percent of the tax cut. That means about two-thirds of the tax holiday was passed along to consumers.

The relief was short-lived. After the moratorium ended, prices rose 4 percent. And the tax cut cost Illinois an estimated $157 million in funds and $46 million was lost from Indiana.

That shows the real cost of putting pennies back in consumers’ wallets.