As of Jan. 20, 1989, Republicans will have held the White House for 16 of the past 20 years. The GOP now holds a virtual lock on the national executive, only slightly less strong than the one the Democrats hold on the House of Representatives, where there are today 12 more Democratic members than there were on the day of Ronald Reagan's first inaugural.

Is there some semi-plausible explanation for this split-level joint custody of the electorate's confidence? Of course there is. According to the polling firm of Yankelovich, Skelly and White, the American public believes that the Republicans are a lot better at those public tasks generally identified with the presidency -- "keeping the economy strong and prosperous," "keeping our defense strong," "keeping inflation under control" -- than are th Democrats. Where the Democrats win the public edge is for their handling of such tasks as "providing jobs for the unemployed," "providing quality education," and "ensuring the rights of women and minorities."

The public trusts the Democrats to nurture and to nourish the body politic. To the Republicans are attributed the characteristics of combativeness and competence.

In American politics, to oversimplify recklessly, the compassionate Democrats are the feminine party and the hard-headed Republicans are the masculine party.

Happily we are moving from a society where roles were assigned to one where roles are achieved. But generalizations persist in our politics where, for many voters, the ideal legislative candidate remains the Republican Woman, a combination that suggests the blend of the tough- minded and tender-hearted. The late Robert Kennedy could safely emphasize compassion in his Democratic presidential campaign because so many critics had already branded him "ruthless." Voters, who are both ruthful and assertive, recognize that we live in a difficult and dangerous world, and therefore prefer national leaders who share that judgment.

Does this semi-plausible theory about the sexual identity of our major parties mean that no Democrat can win the presidency in 1988? Not exactly. But the next Democratic nominee (who must first, of course, negotiate all the organized compassion constituencies in the Democratic primaries) had best be able to produce personal occupational credentials that establish executive abilities.

Having been a governor who balanced an unbalanced budget would help, as would having rescued hundreds of thousands of workers' jobs by bringing a company back from the brink of bankruptcy. Of course, the Republicans might help by selecting a 100 percent, hairy-chested he-man as their standard bearer. But Democrats had better not depend on it.