WHEN RONALD REAGAN was running for president in 1980, he said it was possible to cut taxes without increasing the deficit because the tax cuts would more than pay for themselves in extra economic growth. One of the president's opponents characterized this wishful theory as "voodoo economics."

Now the opponent, Vice President Bush, again a presidential candidate, has proposed a self-liquidating tax cut of his own. He said the other day that he favored cutting the maximum capital gains rate from the present 28 percent to 15 percent. "Our experience . . . has shown that such an incentive would not cost the government money," Mr. Bush intoned. "It would gain additional revenue by stimulating growth."

The vice president was right the first time. We don't mean to be spoilsports; the campaign promise is one of our favorite art forms, and Mr. Bush's latest is by no means the most egregious we have heard even at this early stage in the campaign. But this campaign is different from its immediate predecessors. The difference is the succession of deficits that have doubled the national debt to more than $2 trillion in the past seven years.

The Democrats have been made to feel this. Their wont in past elections was to propose new spending programs. (Sometimes they said these, too, would pay in part for themselves by priming the pump.) They can't make such proposals anymore; there isn't any money.

But surely the same discipline also operates on the tax side of the budget. The administration of which Mr. Bush has been so loyal a part has already spent the next president's funds, or enough of them so that it will be a long time not just before the next spending increase but before the next tax cut as well. Tax increase is much more like it.

In a perfect world it might indeed be fair to tax capital gains at somewhat less than ordinary income, as used to be done. That would be because longer term capital gains especially are made up partly of inflation, which probably should not be taxed. But it is also true that capital gains go mainly to the rich, so that a large cut in the gains tax of the sort the vice president proposes would not only in the first instance cost the Treasury a lot of money it cannot afford, it would also change the tilt of the tax code while reintroducing the very complication around which so many tax shelters were constructed in the past. Those are all the reasons why the distinction between capital gains and ordinary income waslargely abolished in last year's tax reform bill. The preferential treatment of capital gains was one of the main provisions traded away for the lower rates that were that bill's signature.

The vice president's proposal would thus affect the balance and fairness of the tax code as well as diminish its yield. Even as campaign promises go, this one ought to wait awhile