IT USED TO BE the Democrats who led the demagoguery on Social Security. Particularly in the 1982 and 1986 congressional campaigns, they made heavy and unprincipled use of the issue. Now the disease has spread. Jack Kemp has begun to play the game in the presidential campaign.

Mr. Kemp believes that deficits don't matter, or at any rate that it is counterproductive to approach them in the conventional way. Instead of tax increases and spending cuts, he would keep the pedal on the floor and let economic growth restore the government's accounts. It's a marvelously sunny view; no one loses. The last time you heard it was from Ronald Reagan seven years ago, which was before he mortgaged the foreseeable future. Mr. Kemp is not contrite. The theory is still right, he says; only the execution went wrong. He pleads that he be allowed to roll the dice just one more time.

But in Iowa the congressman has now gone a step further. In a mass mailing and by other means, he has begun to attack Republican front-runners George Bush and particularly Bob Dole for their votes in 1985 -- the vice president had to break a tie in the Senate -- to cut the deficit by a combination of tax increases and spending cuts, including a one-year-only suspension of Social Security cost-of-living increases.

In fact, these votes were the high point of the legislative leadership of which Mr. Dole now boasts in his campaign, a heroic effort by Senate Republicans, then in the majority, to overcome both Democratic and administration resistance and haul the deficit out of the danger zone. For a few weeks even the president was aboard, as Mr. Bush's role attests. Then the president reneged.

Mr. Kemp is trying to galvanize elderly Iowans against his rivals on grounds that what they did before, they would do again -- that they are a threat to benefits. On one level that is fair enough; Mr. Dole no more than anyone else is entitled to have it both ways. But the Social Security system is so freighted with myth and fears -- and so central an element in the budget -- that Mr. Kemp ends up doing something more, and a lot less pleasant, than just jabbing in the normal way at rival candidates. He is no longer preaching economic optimism. He is trying to mobilize a vulnerable part of the electorate behind a beckoning feel-good philosophy that, if left in place, will crush us all. It's an ugly way in which to proceed.