It's a great plan. But it won't work.

It won't work because the Jackson problem won't go away even if Jackson decides to run for mayor. The "Jackson problem" is not one radical, provocative candidate preaching the agenda of the poor and oppressed and frightening away middle-class whites. The problem is that the poor and oppressed really exist. And they won't go away, even if Jackson does.

The Jackson problem is not so much a problem as an omen. Those who voted for Jackson we're voting for what Jackson was saying and what other Democrats were not. They were voting for his constant reminder to look inward, to look at our cities, our children, our education--to look and be appalled. They were voting because he was saying what needed to be said when no one else would.

So what's the problem? In a party that has made social welfare programs the bulwark of its agenda, Jackson's insistent criticism of Reagan's failure to promote social justice should be put under the spotlight, not hidden under a bushel.

George Bush was able to pretend to leadership on issues such as child care, education and the envronment simply because few Democrats were claiming otherwise.

This, too, is appalling.

You can think that Jackson was a monomaniac who refused to accept defeat gracefully. You can also believe that other Black leaders who have recently moved up the political ranks--such as newly-elected Governor L. Douglas Wilder of Virginia--have made Jackson's day of "outsider politics" a thing of the past.

But it would be short-sighted and destructive to the Democratic Party to believe that the "Jackson problem" was just that--a nuisance caused by one man.

JESSE Jackson may be the Democrats' best hope to define itself. He may never win the presidency, but he can keep the Democrats on their toes and remind them why they are Democrats.

The Democrats lost 1988 not because they were seen as similar to Jackson, but because they were not seen as different from George Bush. They deserved to lose.

The Democratic Party would do best to select a candidate who believes that Jackson's campaign was not a weakness, but a strength.

After all, a Democratic party without Jackson's agenda would be a party that looked all too much like the Republican party.

And one Republican party is plenty, thank you.