REPUBLICANS AND DEMOCRATS are learning that promising to cut taxes when the government is running $200-billion-a-year budget deficits may not be such a smart idea.

The conventional wisdom in this town is that deficit-reduction is a drag. Nobody gets credit for it. Voters wail like banshees when their favorite programs are cut. So why not just let the deficit ride for a while and cut taxes instead?That seems to have been the philosophy behind the $1.6 trillion budget for 1996 sent to Congress the other day by President Clinton, who figures middle-class tax-cutting is the way to go.

Not only did Clinton abandon his pledge to further reduce the deficit, he did nothing to slow down entitlements like Medicare and Medicaid that are growing at three times the rate of inflation.

In a sense, you can understand why Clinton was reluctant to recommend deep cuts in federal spending. He knew the Republicans, who now control Congress, would have taken the money and given it back to taxpayers, rich and middle class alike.

That's what they promised in their Contract With America. By one estimate, the Republican cuts would cost $205 billion over five years, while the more modest Clinton cuts would cost $63 billion.

The truth is that these proposed tax cuts make little sense from a purely economic point of view - and lawmakers (defying all the rules of politics) are beginning to say so on Capitol Hill.

There will be tax cuts this year. You can bet on that. Some relief will be given to families with children. There is strong support for stimulating investment by reducing the capital gains tax.

But many of the proposed cuts will be dropped. Clinton's tuition tax deduction (up to $10,000 a year) will have rough sledding. So will the Republican promise to roll back taxes on Social Security benefits paid by retirees with substantial incomes.

The question Congress should be asking is this:

Why are we cutting taxes at a time when the economy is thriving and we're adding $200 billion a year to the $4.6 trillion national debt?

No one seems to care about the legacy of debt we're leaving our children and grandchildren.

Sooner or later, popular programs like Medicare and Social Security will run out of money unless presidents and lawmakers stop running for re-election and start balancing the books.