NO WONDER President Reagan has blessed the Senate Budget Committee's new plan. It would raise little more in new revenues than the president himself asked, and it treats defense spending very gently. It differs from the president's original February budget chiefly by proposing to cut all other spending even more severely than Mr. Reagan himself had suggested. It is fundamentally implausible.

Why is it implausible? Because it requires legislation that most of the Senate Republicans, let alone a majority of Congress, would oppose. When the February budget appeared, there was no visible surge of partisan enthusiasm for its prescription to keep sawing away at the nutrition money, or to impose fees on Medicaid patients, or to keep squeezing welfare. Nothing has happened since then to change anyone's mind. On the contrary, the steady rise in unemployment has suggested to a good many senators, not all of them Democrats, that the present moment is not opportune for introducing people in distress to Horatio Alger.

This agreement between the Senate Budget Committee and the White House has not produced a budget but rather a rough sketch for a future budget. It indicates cuts and increases only in the broadest terms. You would be justified in suspecting that its immediate purpose is to get the Senate leadership through two difficult moments in the weeks ahead without splitting the party. The Senate must shortly pass its first budget resolution for 1983--the legal deadline is May 15--and then it must pass legislation raising the federal debt ceiling. To get the votes of the conservatives, the Senate leaders have to pledge allegiance to some sort of scheme to get those deficits down. But they do not want an open fight with the White House over the one issue that is central to this administration.

The serious budget negotiations collapsed a week ago. In lieu of a serious compromise, the Senate leadership will make do for the present with this one. The president's February budget called for a reduction of $31 billion in non-defense spending. The Senate's sketch would reduce non-defense spending by $52 billion.

Unfortunately, the Democrats are apparently going to attack this budget on the one subject on which it is right--Social Security. The outlays for Social Security will rise nearly $20 billion next year, and a reduction of $6 billion in that rise is not unreasonable. Most of it could be achieved by deferring next year's annual cost-of-living increase for three months.

The central fault in this sketchy and, probably, short-lived compromise lies elsewhere. It lies in the senators' acquiescence in the Reagan tax policy, which is leaving the country with too little tax revenue to pay for its basic public responsibilities.