FORTUNE — Poor Mitt Romney. From the GOP primaries, when members of his own party labeled him a “vulture capitalist,” to the start of the campaign proper, when one of his top aides vouchsafed that he would follow an “Etch A Sketch” strategy, to the infamous “47%” video, in which he labeled half the country moochers — his campaign was a case study in unforced errors and futility. Today, though, I come to praise Romney, not to bury him. In failing in his quest to reach the White House, he unwittingly made an important contribution to the country’s future: He discredited fuzzy math.  For decades now, politicians in both parties, but particularly the Republicans, have been promising voters the earth, using arithmetic tricks and outright whoppers to explain why they wouldn’t have to pay for it — why, in fact, they could have their cake and eat it in the form of new tax cuts. As night follows day, higher spending and lower revenues have led to bigger deficits and more debt.  MORE: Obama will tax the rich more  Romney promised more of the same. During the GOP primaries, under attack from his rivals for being too timid, he hastily put together a new economic plan, the centerpiece of which was a 20% cut in income tax rates: The 35% rate would go to 28%, the 28% rate would go to 22.4%, and so on. At the same time, he pledged not to cut a dime from Medicare or Social Security, and to boost the Pentagon budget by up to $2 trillion over 10 years. Asked how he would finance this largesse, he talked vaguely about reducing or eliminating tax breaks, but didn’t specify which and explicitly ruled out tackling some big revenue losers, such as the low rates on dividends and capital gains.  In time-honored fashion, Romney was offering the American voters a free lunch, and why not? It worked for Ronald Reagan, who cut taxes and gave a blank check to the Pentagon; for George W. Bush, who cut taxes, started two costly wars, and introduced a prescription-drug program for retirees; and for Barack Obama, who cut taxes, boosted infrastructure spending, and introduced universal health care. For a while there, Romney’s tax strategy appeared to be working too. But then some nonpartisan authorities, such as the Tax Policy Center, started to question its figures, pointing out that the revenue raised wouldn’t nearly cover the cost of the giveaways.  MORE: Missing from Romney’s tax plan – reality  For once, the voters listened. The tax plan, far from winning the election for Romney, handicapped him all the way to Nov. 6. With even conservative commentators such as George Will saying it didn’t add up, he was forced to issue a series of clarifications — insisting he would neither balloon the deficit nor introduce backdoor tax hikes on the middle class. In the final weeks, he deemphasized tax cuts, focusing on his plans to boost domestic energy production and help small businesses. It was a tacit admission that his effort to flout the laws of arithmetic had failed.  That, surely, is progress — as is President Obama’s insistence that the Bush tax cuts for the very rich be reversed. Going forward, there will doubtless be more presidential candidates who place tax cuts at the center of their platforms. But after seeing what happened to Romney, they will have to do some proper cost projections and specify where the money will be made up — what areas of spending will be cut, which tax breaks will be eliminated, and how much revenue will come from higher growth. Nothing less will be considered credible.  The next step on the road to fiscal sanity is to tackle the shibboleth that in no circumstances, even a looming fiscal crisis, should taxes be raised on middle class people, where “middle class” is defined to include families earning upwards of $200,000 a year. So far, neither Obama nor anybody in the Republican leadership has been brave enough to go that far. But let’s be grateful for small mercies. In the election of 2012 a particularly egregious exercise in fuzzy math failed.  This story is from the December 3, 2012 issue of Fortune.
Like three presidents before him, Mitt Romney told the voters what they wanted to hear about taxes. But this time they weren't buying it.