THE DEMOCRATS' search for a seventh man continues -- but doesn't get very far. Listen to just about any Democratic player these days, and it won't be long until you hear the question raised of whether some strong noncandidate can somehow enter the race. These Democrats blanch when they read polls showing that Undecided is leading the Democratic field, with Jesse Jackson in second place, or when they reflect on how any of their paladins would run today against George Bush or Bob Dole. There is a yearning in some quarters for Sam Nunn and Bill Bradley, both of whom have said they're not running, and many Democrats join Doonesbury in Waiting for Mario.

Can anyone who isn't running now win? Can a seventh candidate come in after Iowa, New Hampshire and Super Tuesday, and win the nomination?

Theoretically, the answer is yes. Democratic delegates and conventions legally can do whatever they want. In practice, the answer is that it would be hard and gets more so every day.

One reason is that favorite son candidacies -- the old standby of politicians who wanted to stay loose and choose the nominee at the convention -- are pretty much out. Last week Maryland Gov. William Donald Schaefer was unable to put together a favorite son slate; this week New York Sen. Daniel Patrick Moynihan abandoned efforts to do so. These two men are among the strongest Democrats around; if they can't do it, probably no one can.

That means that the delegates, under the Democratic rules, will be picked entirely by the candidates. It's possible that a sizable number of delegates will be freed up by their candidate's withdrawal or that delegates will abandon their candidate's cause, but not likely. Any dealing will probably be done by candidates, and it seems likely they'd choose one of themselves, not someone else who hasn't been running.

In the meantime, the timetable hurts late entries. Some 45 percent of the Democratic delegates will be chosen, and the filing deadline for 78 percent of the delegates will have been passed, on Super Tuesday March 8. It's still possible to write seventh-candidate scenarios, but it takes an extravagant imagination to see how they can wor