How would you like to be a leader of the Democratic Party forced to decide whether to support a Jesse Jackson run for the presidential nomination?

The pluses would be obvious. A Jackson candidacy would guarantee a tremendous surge in black voter registration, and more than 90 percent of the new registrants would be Democrats. You'd assume that Jackson wouldn't win the nomination, of course, but that block of previously unregistered Democrats might make the crucial difference for the Democratic nominee in the general election.

After all, Ronald Reagan's 1980 victory margin in all but one of the southern states was substantially less than the number of unregistered blacks. If a significant number of those blacks had voted, Jimmy Carter would still be president.

On the other hand, there's the possibility that while Jackson couldn't win the party nomination, he might succeed in taking so many votes away from the more liberal front-runners that the party convention could end up nominating some conservative dark horse. In such a case, black voters, though registered, might have little incentive to vote in the general election, and they just might feel so betrayed that they would say to hell with the party. Such an outcome could leave the Democratic Party worse off than it is now.

If you are attracted by the pluses of a Jackson candidacy, you might be tempted to make some Democratic money available to him. Jackson's most obvious weakness is his doubtful ability to finance an all- out run in the primaries.

If you are worried over the minuses, and worried, too, that a Jackson run might so "blacken" the party platform as to make the eventual nominee unelectable, you certainly wouldn't want to finance his effort, and you might even work to discourage it.

It's a nice little puzzle that, interestingly enough, has virtually nothing to do with the ambivalence (even hostility) of the national black leadership toward a Jackson candidacy.

Their public statements focus on the advisability of "a black" running for president, and whether it might improve Reagan's chances for reelection. Their more important misgivings, however, may stem from the fact that the only black remotely in position to make such a run is: Jesse Jackson. If the prospective black candidate were an Andrew Young or a Julian Bond, you might see a markedly different attitude among the black leadership.

Jackson, they freely acknowledge, has the ability to excite the black masses to political involvement. They concede that he is bright, bold, charismatic and a keen analyst of issues. It's hard to find a member of the black leadership who will take issue with anything Jackson has to say (except, perhaps, for his all-but-forgotten boycott of Anheuser Busch).

Their problem with Jackson is not what he says and does, but why he says and does it. Their suspicion is that Jesse Jackson's overriding interest is in promoting Jesse Jackson. Part of their attitude can be chalked up to jealousy. But another part is that Jackson operates without effective control, either from his own organization, Operation PUSH, or from the black leadership "family."

What both black leaders and Democratic Party leaders would dearly love to see, for quite different reasons, is a Jackson-led get- out-the-vote campaign--on behalf of someone else.