For decades, ever since Ronald Reagan was elected in 1980, promising to cut taxes has been an essential element of every successful Republican campaign for the presidency. Republican politicians still vividly remember that George H.W. Bush lost to Bill Clinton after agreeing to a modest tax increase as part of a broad budget deal — and most have vowed never to make that mistake again.

But there is a crucial twist to the campaign this time around. All of the Republican candidates have pledged to extend President Bush’s tax cuts from the early 1990s beyond their scheduled expiration in 2010. That promise, however, does not carry the same weight as in the past.

That’s because, rather than delivering any additional benefit that voters can actually take to the bank, carrying out such a pledge would do nothing more than maintain the status quo. Nobody’s taxes would be cut further; they would at best stay the same. There’s not as much political payoff in that.

And preventing anybody from being worse off is going to be incredibly costly. Indeed, it requires running faster and faster just to stay in the same place. A new report from the Congressional Budget Office on the long-term budget outlook, delivered to Congress on Thursday, makes clear the depth of the fiscal hole the next president will inherit from President Bush.

Simply to extend the Bush tax cuts indefinitely into the future and, as both Republicans and Democrats have vowed, prevent the alternative minimum tax from imposing an increasingly heavy burden on tens of millions of middle-class and upper middle-class taxpayers would cost the government, over the next decade, roughly $2.5 trillion in revenues now expected under current law. And that’s just the beginning.

Even without taking on any additional tasks, merely meeting the government’s existing obligations — mostly to pay for the military and to keep up with the health care and retirement needs of the elderly — would send the budget deficit soaring, pushing overall federal debt held by the public from under 50 percent of the size of the nation’s economy today to over 300 percent by 2050.

“The combination of roughly constant revenues and significantly rising expenditures would quickly create an unstable fiscal situation,” the budget office report notes alarmingly, but in its characteristically dry and understated manner.

How would the Republican candidates deal with this problem? Most say they would try to hold down spending — and cut taxes even more.

Indeed, without providing many specifics about his proposed spending cuts, Rudolph W. Giuliani, in a recent op-ed article in The Wall Street Journal, wrote that he was “committed to making the 2001 and 2003 tax cuts permanent, while aiming at still-lower marginal rates. We’ll give the death tax the death penalty, index the Alternative Minimum Tax for inflation as a step toward eliminating it entirely, expand tax-free savings accounts, and expand health-care choice through tax reform. We also need to reduce the corporate tax rate.”

Fred D. Thompson recently unveiled his own tax proposal, which would not only match the Giuliani promises, but would also allow taxpayers to choose between paying under the current system or opting for a “flat tax” with lower rates that would eliminate nearly all deductions. The simplified tax system would have just two rates: 10 percent and 25 percent.

The nonpartisan Tax Policy Center analyzed Mr. Thompson’s overall proposal and found that it would “represent, by far, the largest tax cut in history — much larger than the tax cuts enacted in 2001 or 1981. Over 10 years, individual income and estate taxes would fall by about $6 trillion to $7 trillion — or as much as 20 percent of overall revenues — before allowing for any behavioral responses.”

Mr. Thompson predicted that the tax cut would largely pay for itself by stimulating economic growth and discouraging tax avoidance. If not, he suggested, any additional savings could be achieved by limiting Social Security benefits.

But the Tax Policy Center report found that any improvements to the economy from lower tax rates would be modest. As a result, the Treasury would recover no more than about $1 trillion over the decade, resulting in an overall revenue loss of $5 trillion to $6 trillion. The tax cuts would fall far short of paying for themselves.

And nearly all the money, like the earlier rounds of tax cuts this decade, would flow to those at the top of the income ladder.

Meanwhile, Mike Huckabee has proposed yet a third alternative, endorsing the so-called “fair tax,” which vows to replace all federal revenues — income taxes, payroll taxes for Social Security and Medicare, estate taxes, etc. — with a national sales tax on everything except education.

Proponents say that a sales tax rate of 23 percent on just about all goods and services would generate the same revenues as the current system, but tax experts like Bruce Bartlett, a former Treasury official under President Ronald Reagan, say that it would effectively mean raising the cost of everything people buy by at least 30 percent.

And even if such a tax could be practically instituted, it would still not close the fiscal gap that is about to explode over the next few years.

“Campaigns bring out the Santa Claus in politicians,” said Leonard Burman, director of the Tax Policy Center, which is associated with the Brookings Institution and the Urban Institute. “But the numbers just don’t add up. By promising more tax cuts than we can afford, they are really misrepresenting the choices the nation faces.”

Democrats certainly have their own problems balancing their spending proposals — particularly for health care — with the revenues available to pay for them, but the Republican candidates, by vowing to extend President Bush’s tax cuts, have left themselves with a far bigger fiscal gap to fill.

So before the Republicans make any new tax promises, it might help if they first told voters how they plan to pay for the old ones.

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