DON'T JUDGE US by what we say but by what we do. Look at the bottom line.

Richard Nixon made this admonition 15 years or so ago, and - especially with regard to the budget debate now getting under way - it's as apropos today as it was then.Much will be said and written about the Reagan administration budget, which was sent to Congress Monday. The critics contend:

* That its priorities are wrong. The Democrats who now control the Senate as well as the House of Representatives, will press to cut the growth in real defense spending - which the administration proposes to hold at 3 percent - even further. In its place they will seek to restore some of their favored social programs.

* That it is unrealistic. Pinning hope on selling assets like Amtrak's money-losing Northeast Corridor is naive or deceptive.

* That it is unimaginative. Most of the cost-cutting ideas are warmed-over versions of proposals that have been made in the past and rejected.

* That it omits any mention of a tax increase, and some increase in excises or import taxes is almost certain, despite the administration's opposition.

* That the whole budget effort is a charade, an exercise in the ridiculous. Congress, not the administration, will be writing the budget for the next two years and administration initia tives, even the most imaginative, are likely to get short shrift.

All of this pales, however, beside the fact that in the current fiscal year - the one ending next Sept. 30 - the administration and the Congress must share blame for a budget deficit hopelessly greater than the level they committed themselves to achieve. If the Congressional Budget Office estimate is correct, the shortfall would be $174.5 billion. If the administration estimate is right, it would be only slightly less, $173.2 billion.

This compares with the $144 billion Congress and the president pledged themselves to hold to under the Gramm-Rudman resolution and the $151 billion Congress thought it had accomplished when it adjourned in October.

There are reasons, of course, for the performance failure. There always are. Primarily, the administration was more optimistic in its economic forecast than it should have been. It said real growth last year, that is economic growth with inflation factored out, would come to 4 percent. Actually, it was only 2.4 percent.

The administration is predicting real growth of 3.2 percent this year and 3.7 percent in 1988, forecasts already in doubt because of the slow response of the international payments deficit to the decline of the dollar. By way of contrast, the CBO is saying real growth will be no more than 3 percent either this year or the next.

Even the CBO sees no likelihood of the deficit being reduced fast enough to meet Gramm-Rudman targets. While it forecasts a continued decline in the budget deficit, the drop would be only to $169.2 billion next year and $162.2 billion in 1989. The budgetary shortfall would drop to $84.8 billion in fiscal 1992 if - and this is a big if - the economy manages continued, if slow, growth.

Doubtful economic forecasts are only one of two very large question marks overhanging budget performance. The other, especially important this year, is the reliability of tax collec tions.

With tax reform going into effect in part Jan. 1 and in part six months later, it's anyone's guess what actual collections will be. The surprises here could be greater than those with regard to the economic assumptions. Supply siders, of course, are predicting that lowering rates plus eliminating shelters and loopholes will bring a great boost in revenues. Others are predicting that the initial effect at least will be a reduction.

This leaves spending as the only reasonably stable measure of budgetary performance. A steady reduction in spending as a percentage of gross national product would be a far better yardstick than the overall budget deficit. Perhaps some sensible person in Washington will propose it.

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