Depending upon your persuasion, Exxon Mobil's $10.9 billion quarterly profit report on Thursday was either a wash-up or a windfall. Wall Street saw the former, questioning why the country's largest oil company couldn't grow its earnings by more than 17 percent while oil sells for more than $100 a barrel.

Both Democratic presidential candidates Barack Obama and Hillary Clinton see profits like Exxon's as excessive, and would subject them to a "windfall profits" tax to pay for programs to help Americans cope with high fuel costs.

Nothing, of course, makes a conservative economist fly off the handle like soaking wealthy oil producers. But even liberals would have a hard time defending the country's last experience with a windfall tax, in 1980.

What began as a compromise by the Carter administration to lift ceilings on oil prices grew into a bureaucratic nightmare that Congress in 1984 called the "largest and most complex tax ever levied on a U.S. industry." The law produced nowhere near the revenue it promised, made the country more reliant on foreign oil, and generated reams of red tape, according to a 2006 report by the nonpartisan Congressional Research Service.

The law was put out of its misery in 1988, two-and-a-half years before it would have automatically expired.

"It's a terrible idea today," said Phil Verleger, who helped design the windfall tax policy as the Treasury Department's director of domestic energy policy from 1977 to 1979. "The windfall profit tax was a quo for a quid; the quid was price decontrol. There's no quid right now."

Neither Democratic candidate has provided enough detail to make a full analysis of their current windfall-tax proposals.

Senator Barack Obama's campaign says he "supports imposing a windfall-profits penalty on oil selling at or over $80 per barrel" and spending the proceeds to help Americans deal with high energy prices.

Senator Hillary Clinton says she'll tax "windfall profits"—no figures are provided on how the term is defined—to pay for a temporary suspension of the 18.4-cent-per-gallon federal tax on gasoline.

Proposals like these have appeared regularly, always when oil prices have spiked.

"Everyone gets frustrated and is looking for a simple answer and a scapegoat, and the oil companies are really easy to vilify," says Gilbert Metcalf, a Tufts University economics professor and a research associate at the National Bureau of Economic Research.