THE NOTION THAT the Democrats have to come up with "new ideas" is the shrewdest Republican ploy since the nomination of Ronald Reagan.

As long as Democrats allow themselves to be measured against a new-ideas yardstick, they're bound to appear miserably inadequate. And as long as Democrats envy how intellectually well-endowed the Republicans seem to be, they won't notice that what they are moping about is a piece of political trompe l'oeil.

There's no question that the Democrats took a drubbing in 1980. But it wasn't the Republicans' new ideas which beat them, and it won't be Democrats' newer ideas which will return them to power in 1984. Yet this is the analysis that now dominates Democrats' thinking. From Kennedy and Mondale to Hart and Tsongas, from the Democratic National Committee to the post-debacle PACs, the loudest common theme is the need to replenish the party's depleted intellectual capital.

Homage is always paid to traditional Democratic values -- social justice, compassion, human rights -- but almost invariably the conversation in these circles turns to the search for a Democratic version of supply- side economics or of the American Enterprise Institute.

To be sure, both the Democrats and the nation stand to gain from the party's efforts at fresh thinking. And there's plenty going on. On the Hill, for example, the Wirth-Gephardt economic policy group is looking at tax policies that would move capital into high-tech sunrise industries. At the new Center for Democratic Policy, an incomes policy tying wages to profits may be a way out of what economist Daniel Mitchell calls the "game of economic chicken" between the Fed and American workers. Hart's "better is better" approach to defense has put Atlantic editor James Fallows' work into political circulation, and Tsongas's "new liberalism" has helped translate MIT Prof. Lester Thurow's zero-sum concept into legislative terms.

What's more, there's nothing wrong with Democrats owning up to their failures. Being against government waste hardly constitutes a new entry in American politics, but when a Mondale and a Kennedy include it among their themes, it's sure to make program efficiency a more acceptable topic among Democrats to the left of the Boll Weevils. The same is true of what Mondale is saying about crime and Kennedy is arguing about deregulation. If only Nixon could go to China, perhaps only a liberal can go to Middle America.

But saying mea culpa isn't how you win elections. It's a way to win back voters' trust, but that only means you've risen from the negative zone to ground zero. Nor is the ferment on the new-ideas front going to win back the White House either, and for two reasons.

First, Democrats are too bedazzled by the Laffer curve, the Heritage Foundation, and all the othe ionography of the Republican victory. Republicans have managed to convince them that their little magazines and think tanks have been churning out something different from the cranky right-wing nostrums they've been pushing for years. Democrats have accorded intellectual gravity to a pack of parvenu lightweights instead of treating the supply-side phenomenon as the economic equivalent of People magazine.

Of course there are serious thinkers in the Republican fold, too. But they had virtually nothing to do with achieving the Reagan victory. The number of voters swayed by what they read in Commentary magazine can be comfortable accommodated in the cafeteria of the City College of New York.

It's fine to appeal to the intellectual vote, and doubtless many "opinion makers" -- from editorialists to Agronsky fans -- would appreciate some attention. But American elections aren't referenda on the quality of candidates' ideas, nor was it ideas with which the Republicans won. They came to power on disgust with the Carter record plus an anthology of old slogans for which "new ideas" seems hardly the right title.

You don't need to convene symposia to come up with "getting government off the people's backs." You don't need a brain trust to invent "family, work, neighborhood, peace and freedom," or "Are you better off now than you were four years ago?" What you need is an affable candidate with a smooth machine, not a Hoover Institute -- or its Democratic simulacrum.

Second, by their almost puppy-like eagerness to come up with new ideas, Democrats lend too much credence to the main Republican charge against them -- that their old ideas were bad.

For example, when Fortune magazine's Daniel Seligman wrote not long ago about "The Search for a Liberal Agenda," here is the breezily confident indictment of Democrats with which he led off:

"The hundreds of millions spent by the Law Enforcement Assistance Administration don't seem to have improved law enforcement. The billions devoted to the Alcohol, Drug Abuse, and Mental Health Administration have had no measureable effect on the incidence of alcoholism, drug abuse, or mental illness. The Administration for Children, Youth, and the Family doesn't know how to restore broken homes. The scores of billions lavished on the Office (now Department) of Education haven't prevented a steady decline in educational achievement. It is debatable whether the National Highway Traffic Safety Administration has increased highway safety. The Occupational Safety and Health Administration has no data showing that its labors reduce on-the-job accidents."

This version of the New Deal/Great Society legacy is what candidate Reagan has been saying for years. Now it's become conventional wisdom, as though all of its contentions were above reproach. As long as Democrats let the Republicans' comic-book version of their past become official history, the more will they seem unfit ever to govern again.

It's one thing for Democrats to admit they spent too much time passing programs and not enough running them well. It's quite another to concede that the Democratic past amounts to screwball social engineering that never could have worked, no matter how effectively it might have been administered. Republicans have seized control of the American memory and have met little Democratic resistance. As a result, Democrats have been saddled with a history they're supposed to repudiate. If they hadn't permitted Republicans to portray their record as a washout, Democrats would today be feeling a lot less feverish about the need to come up with new ideas.

If Democrats choose to be judged on whether they've come up with new ideas, they should at least realize that they've allowed the Republicans to set the terms of the contest. The party that immortalized a curve drawn on a cocktail napkin has conned the Democrats into a game they can't win. But sharp politicians know they should never fight on turf they haven't chosen themselves.

To get back on native ground, Democrats should take Republicans' ideas less seriously. For example, in the 1978 congressional races, the Kemp-Roth tax cut was laughed off the campaign trail. Now that it's law, it should be treated as a cockamamie scheme the way it once was -- and not as a bold intellectual breakthrough requiring a commensurate Democratic counter-idea.

What new ideas the Democrats do develop may well be just the policies the nation needs. But however salutary they may be, they're highly unlikely to lead to electoral victory. For that, a Reagan failure is necessary, plus some smart political jockeying of the most traditional sort.

In private, when they're not issuing statesmanlike appeals for new ideas, Democrats admit that this is what it all comes down to. Walking past the White House after a National Press Club luncheon where three economists had laid out some ideas under Center for Democratic Policy auspices, one well-known Democrat connected with the Center acknowledged that he'd just heard nothing new. Nor did that bother him.

"There was nothing new in the ideas that brought Reagan to power, either," he explained. "The point of all this isn't to come up with new ideas. The point is to have the best blue smoke and mirrors."