Newt Gingrich, meanwhile, has promised us $2.50-a-gallon gasoline. But if we can suspend the law of supply and demand, why stop with gasoline? Why not $2.50 for one-carat diamonds, steak dinners and 18-year-old Scotch whiskey?

Although the United States cannot unilaterally lower the price of oil, it can reduce its consumption, by using oil more efficiently and by developing alternative sources of fuel. For example, the Obama administration has raised the corporate average fuel economy standards imposed on automakers. If consumers buy more fuel-efficient cars and trucks, demand for gasoline falls, as does the burden imposed by high gas prices. But while such rules help, they are not the best way of achieving societal goals.

A better approach would be to gradually raise the gasoline tax to levels similar to those in Western Europe, where fuel-efficient cars are the norm. N. Gregory Mankiw — the Harvard economist who advises Mr. Romney and is a fellow contributor to the Economic View column — has long advocated such a policy. I agree with him, as do most other economists.

For evidence, note that the economists in that same University of Chicago poll were asked whether they agreed with this statement: “A tax on the carbon content of fuels would be a less expensive way to reduce carbon-dioxide emissions than would a collection of policies such as ‘corporate average fuel economy’ requirements for automobiles.”

On this question, there was just a single negative vote. Yet in our current political climate, making the sensible suggestion that we gradually raise the tax on gasoline — or impose a broader system of carbon taxes — is ridiculed, and no one running for president can safely make such a proposal. At least two of the candidates have shown that they understand the underlying economics. In the past, both President Obama and Mr. Romney have acknowledged that higher gas prices have an upside: they give car owners the right incentives, and if the high prices stem in part from higher fuel taxes, the deficit can be trimmed. But such obviously true statements are now considered almost unpatriotic, equivalent to cheering against the U.S.A. in the Olympics.

THE confused public debate on this topic is representative of a more general problem. The voting public is not very good at attributing credit and blame to presidents. They get too much credit when things go well and too much blame when things go badly. The same applies to coaches, C.E.O.’s, parents and anyone else in charge. Leaders are important but not omnipotent.

So, to evaluate a leader, we must determine the factors over which that leader has a modicum of control. If you hate the Obama health care program and the Consumer Financial Protection Bureau, by all means give the president a big share of the blame. And if you love them, give him some credit. What makes no sense is to blame the president for rising gas prices, where he has virtually no control, but not to give him some credit for rising stock prices and an improved labor market, domains where his policies — along with those of the Federal Reserve and Congress — are more likely to have had an effect.

When we make our choice on Election Day, we should consider that the winner will have an important impact on policies in many areas: health care, distribution of the tax burden, Supreme Court nominations, and abortion rights. The candidates’ differences on those issues should be driving our decision, not the wishful thinking that a president can simply lower the price of gasoline. Or Scotch, alas.