The exercise of clemency -- reducing a sentence determined by a judge or jury, freeing someone convicted of a criminal offense -- is one of the most sensitive functions of a governor or president. It is one kind of serious mistake to exercise clemency when it is not warranted and another not to when it is. Yet there are no hard- and-fast rules for an executive to follow. He has to learn the facts, master the record and make his best judgment.

Something like the opposite procedure seems to have been commended by George Bush. "I can tell you one thing about the difference between a liberal politician and a conservative one," Mr. Bush said Thursday at a Conservative Party dinner in New York, "Gov. Ronald Reagan kept cop killers in jail."

The reference was not lost on the audience. New York's Gov. Mario Cuomo recently recommended parole for Gary McGivern, a convict who has served 18 years for the felony murder of a prison guard. Among others, writer William F. Buckley Jr., who has studied the case, believes parole should be granted. But the parole board denied it. All 56 Republicans in the New York Assembly oppose it. The White House is reported to have urged Republicans to attack Gov. Cuomo for his decision.

At a minimum it seems to us that the judgments of Mr. Cuomo and Mr. Buckley are based on detailed knowledge and extensive research and deserve respectful notice. Does George Bush have a similar basis for suggesting that this decision for clemency is an abuse of Mr. Cuomo's discretion? In 1967 then-governor Ronald Reagan, at the recommendation of Edwin Meese, reduced from death to life imprisonment the sentence of Calvin Thomas, who was convicted of murdering his girlfriend's baby after he hurled a firebomb into her home. Would it be fair to contrast Mr. Reagan with another governor "who kept baby killers in jail"?

Mr. Bush must know the answer to that. The New York remark isn't like him. Is this what's in store for '88?