When the characters are too close to their real-life prototypes they have no rounded reality of their own.

The roman a clef is an accepted form of fiction, but it has its tricky aspects. So is the parallel between Nancy Reagan and the unsympathetically portrayed Prilla, ''sixty if she was a day,'' who knows that there isn't a mortal thing she wouldn't do to get her husband to the White House. (The gritty scenes of her battles against her addiction are among the best in the book.) The intended likeness to Joan Kennedy is, of course, unmistakable. Robin is red-haired, an amateur but gifted painter and a sobered alcoholic whose political reconciliation with her husband threatens her sobriety and stability. Breslin plays out the situation from the points of view of the First Ladies of the title - the temporary First Lady, beautiful Robin Devlin, and the potential First Lady, Priscilla (called Prilla) Sterling. The intriguing possibilities inherent in this constitutional crisis and the Vietnam-Cambodia scenario are, it seems to me, somewhat vitiated because Ms. His incumbency is temporary but long enough to bring us to the brink of negotiations with Vietnam to salvage a ravaged Cambodia. Following the provisions of the 12th and 20th Amendments, Devlin becomes acting President. The Democratic Senate, meanwhile, chooses Devlin as Vice President by a simple majority. In accordance with the 12th Amendment, the House of Representatives must now choose the President, each state delegation having one vote. The Republican contender, former Governor Dan Sterling, who has come to politics by way of Hollywood, wins the popular election by two million votes but his ticket fails to get the necessary majority in the Electoral College.

Democrat Roy Bob Talbott is the incumbent President his running mate is Senator Larry Devlin of the famous Devlin clan, which has already produced a 20th-century President. The candidates wear thin fictional disguises. Catherine Breslin, the author of the novels ''Unholy Child'' and ''The Mistress Condition,'' has managed this unlikely feat in an ingeniously plotted story. TURN back the clock to 1980, indulge the fantasies of the loyalists who long for a return of those ''born American royal'' to the White House, and still come to terms with what actually happened.

Unholy Child by Catherine Breslin