Let's all go down to the National and cheer Roosevelt. Or hiss Roosevelt. Whichever the thought of Franklin D. Roosevelt promised you to do before will only be intensified by Dore Sharys new one-person play, "FDR".

This is purely an evening of exposure to FDR's famous charm, deftly reproduced by Robert Vaughn. Probably, as with the voters of the '30s and '40s, most will be enthralled and a minority repelled.

There are no historical or salacious revelations in "FDR", no attempts at hindsight analysis in terms of his illness, his wife of him mommy. There are no moral conflicts, no character-revealing struggles. A happy and confident President leads his official life as it was publicly documented at he time.

Some of these omissions are tremendous virtues, hardly to be expected in modern biography. The entire Lucy Mercer affair is covered in one sentence, in which FDR tells his daughter he will not discuss it, that her mother "knows and understands everuthing," and that is enough. This, with his simultaneous affection of Eleanor, seems more dignified and characteristic than any smoldering drama of passion and resentment would be.

But because there are no issues in this play, there is no opportunity to regard its hero as anything but an overpowering "personality." When, for instance, his attempt to pack the Supreme Court comes up, it must be taken as you take him - as charming and bold, if the play has worked its spell. There's no room here to accept the man and still condemn any move he made.

Roosevelt's delayed-action grin may seem captivating to people who were repelled by Nixon's suddenly-remembered television smile. "Mertom, Burton and Fish" may seem a witty phrase to people who disliked Agnew's allterations. Roosevelt's telling the Daily News editor that his editorials are getting American killed may seem patriotic to people who found Nixons denundations an abridgement of freedom. The Fala speech may be a delight to those who found the Checkers speech reprehensible.

You cannot use your judgment about this character. But you can have a marvelous time cheering him - or hissing.