Economic theories come in many flavors. There are supply-siders and deficit hawks, monetarists and Marxists. As Richard Nixon once said, and the recent bipartisan bout of economic pump-priming proved, "We are all Keynesians now." John McCain has devised yet another approach to the dismal science -- "that was then, this is now."

In the space of just a few years, McCain has morphed from someone who worried about the cost of the Bush tax cuts into a rabid tax-cutter. You don't need a fancy equation to explain this turnabout. McCain is running for president at the helm of a party that's deathly allergic to taxes and highly suspicious of him on this score.

McCain 2001 said he could not "in good conscience support a tax cut in which so many of the benefits go to the most fortunate among us, at the expense of middle-class Americans." McCain 2008 pushes a tax policy that makes Bush's plan look like a soak-the-rich scheme. For example, McCain says repealing the alternative minimum tax would protect the middle class. In fact, getting rid of the AMT -- as opposed to patching it to make sure it does not catch increasing numbers of taxpayers -- would primarily benefit those with annual incomes of $500,000 or more.

McCain 2003 correctly called additional tax cuts "irresponsible," adding, "At a time of war, at a time of economic stagnation, at a time of rising national debt . . . one might expect our national leaders to pursue policies calling for shared sacrifice to achieve shared benefits. Regrettably, that is not the case." Regrettably, McCain 2008 has junked "shared sacrifice" in favor of tax cuts that the nonpartisan Tax Policy Center says would total $553 billion in 2012. Of that, $179 billion comes from extending the Bush cuts he once opposed and more than twice that amount -- $374 billion -- from McCain's new ones.

Are spending cuts of that magnitude conceivable? Are tax cuts of that size responsible? Let's take as our guide a conservative, Douglas Holtz-Eakin, former chief economist in the Bush White House, former director of the Congressional Budget Office, and -- at least until his current incarnation as McCain's chief economic adviser -- a model of economic sense.

Just a few years ago, Holtz-Eakin was offering, well, straight talk about the feasibility of cutting spending anywhere near enough to cover existing costs -- not new tax cuts such as those his candidate is now proposing. "While everyone is in favor of eliminating waste," he wrote in the Washington Post in February 2006, "Americans want to protect defense, education, environment, transportation and a myriad of other programs. Congress expended a tremendous effort last week and just barely adopted $39 billion in cuts from projected spending -- less than 7/100ths of 1 percent of GDP."

Now, Holtz-Eakin airily tosses off promises to cut $60 billion in discretionary spending "that traces its existence to earmarks" -- as if that's a sensible way to distinguish the necessary from the pork.

Just a few years ago, he was acknowledging that new taxes -- not hundreds of billions in additional tax cuts -- would be needed. The "current level of taxes will fail to cover the type of government Americans have grown used to," he wrote. "That's bad news because the government will not be getting any smaller. . . . I'd rather not raise taxes, but unless government remains at its traditional size, I don't see any way around it."

• Ruth Marcus is a columnist for the Washington Post Writers Group.