THE G.O.P. LITMUS TEST

''A day that will live in infamy,'' declared TV Evangelist Pat Robertson, referring to the Supreme Court's reaffirmation last week of Roe vs. Wade. Citing Thomas Jefferson, the Republican presidential aspirant called the high court an ''unelected oligarchy'' and assailed the Justices as ''despots.'' Some 1,200 delegates to the National Right to Life Committee's convention in Denver applauded warmly. Then came New York's Republican Congressman Jack Kemp, a more conventional politician and a virtually certain candidate for the 1988 presidential nomination. Kemp took a broader view, shunning personal attacks on the Justices and appealing for ''not just a change of law but a change of heart'' on abortion. His listeners responded with ovations that surpassed the reception given Robertson.

Finally, Senate Majority Leader Robert Dole aroused the delegates, predicting, ''The fight can be won if we continue to press for the appointment of judges who interpret the law rather than invent the law.'' The appearance of Robertson, Kemp and Dole in Denver signaled that abortion has become a litmus-test issue for right-wing support in G.O.P. presidential politics. By coincidence, the three-day right-to-life meeting took place only five blocks away from a hotel that was host to the annual convention of the National Organization for Women, one of the most vocal advocates of free choice. The two groups avoided any serious confrontations. The social agenda of the New Right has considerable clout within the G.O.P., and the right-to-life movement's passionate activists carry far more weight in the party's nomination struggle than their numbers would indicate. So the main Republican presidential hopefuls were already pledging their antiabortion allegiance. But where was George Bush? The Vice President had a good claim to urgent duties in Canada, where he presented the U.S. position in negotiations on freer trade. J.C. Willke, president of the National Right to Life Committee, at first accused Bush of reneging on a promised Denver appearance but then met him amicably in the White House and withdrew the accusation. Willke even read a greeting from Bush to the convention. The antiabortion positions of the prospective Republican candidates differ more in emphasis than substance. Robertson and Dole want to return to the pre- Roe status, letting each state decide what limitations to place on abortions; they assume that most will restrict them. Kemp backs an amendment that would permit abortions only in situations where childbirth endangered the life of the mother. Bush would add rape and incest to the circumstances under which abortions could lawfully be performed. Inescapably, Bush is linked to the actions of President Reagan, who has consistently opposed abortion. But Reagan has not satisfied the right-to-life movement's most ardent activists, who feel he has failed to deliver any effective antiabortion legislation. Paul Brown, chief executive officer of the American Life League, a smaller antiabortion group, implicitly conveyed the passions that the issue arouses. ''Our faith in President Ronald Reagan,'' he declared, ''may have been the biggest mistake in the history of the pro-life movement.'' As the 1988 presidential campaign gets off to an early start, the abortion issue seems destined to play a divisive, and possibly even decisive, role in it.