Sen. Ted Kennedy's withdrawal from the 1984 presidential race has been greeted by most observers as a mercy to his family and party: for one, a respite from fear; for the other, a respite from what my colleague David Broder correctly calls "soap opera."

Sharing these sentiments, I was about to let the occasion pass when Arthur Schlesinger Jr., the official Kennedy hagiographer, struck a different note.

While the senator's decision demands "forbearance and respect," writes Schlesinger, it "impoverishes the national debate." Think what one will of Ted Kennedy, this is the first time he has been confused with Webster, Lincoln and Douglas. (For the full elaboration of this startling assertion, see Professor Schlesinger's piece in the Dec. 7 Wall Street Journal.)

In brief, his thesis is this: Ronald Reagan, though a failure, has at least "redefined the terms of the national debate" (that debate again!). Only Kennedy has frontally and consistently challenged the main theories of Reaganism. Now that Kennedy has quit the field, his pronouncements will lack that "political pressure" that an active candidacy generates.

Hence, impoverishment. No other Democratic candidate, says Schlesinger, now comes near opposing Reaganism as an opposition should. Others, especially the wicked "neo-liberals," merely parrot the Reagan tune in a different key.

Well, that's one way to look at the political scene: the storybook way. There is another. Unlike Schlesinger, I find "Reaganism" and the alternative message preached by Kennedy to be cousins beneath the skin. Both are interdependent varieties of political showmanship of doubtful applicability to national problems.

Both Reagan and Kennedy are masters of the political theater that makes partisan juices flow. Both make a great set speech; both offer an uncompromising vision. The president's nostalgia for the unregulated marketplace and the low tax rates of the '20s makes Kennedy's nostalgia for "affirmative government" (as Schlesinger delicately calls it) appealing and plausible.

But how much does either vision have to do with political reality? Something, maybe, but you couldn't prove it by results.

The president, before his election, said the answer to inflation was a balanced budget; and for recession, a radical income tax cut. His actual policy has given us a tide of red ink, while his tax cuts have done nothing worth noting, so far, about the recession.

While Reagan was extolling his sovereign remedies in the 1980 campaign, Kennedy was marketing a sovereign remedy of his own: price and wage controls. These, perhaps, are what Schlesinger means by "structural change," otherwise undefined.

On the surface, these views are indeed antithetical, and you can have a merry old argument about which are preferable, if you prefer either. I don't. Richard Nixon, the last president to apply direct price and wage controls, did so when the inflation rate was a manageable 3 or 4 percent. That was almost the last we saw of inflation so modest. Perhaps Kennedy knows where the votes are in Congress for this dubious measure. One suspects they're as rare as the votes for deficits on the Reagan scale.

Much the same can be said of the Reagan-Kennedy "debate" on foreign policy. Kennedy's trump card, the nuclear freeze, would leave unredressed the challenge to NATO of Soviet SS20s. It is no more satisfactory than the "remilitarization" of the Cold War with which Schlesinger charges Reagan.

Who, then, is impoverishing the national debate? Perhaps it is those who offer plausible ideological solutions to complex problems. What help does either the Reagan or Kennedy approach offer for the alarming decline of the competitiveness of basic industry? For the incapacity of Congress to get the budget in hand? For the failure to stimulate stable growth without inflation, or price stability without recession? For the nuclear-arms stalemate?

If formulas to meet these needs are to be found in exciting and entertaining flourishes, it will be a surprise to most of us. Back to Andrew Mellon, or forward to the utopia of "affirmative government" and "structural change," it looks an awful lot like political spinach. Horse opera or soap opera, a little of this goes a long, long way.