THERE'S NOT much point, we think, in speculating about the political effects of former president Jimmy Carter's endorsement of his former vice president, Walter Mondale, for president in 1984. Endorsements don't sway many votes in presidential elections; by November voters know a lot about the candidates and make their own decisions. Yes, Mr. Carter still has some enthusiasts. But even the most popular former president--and there haven't been any for a long time now--can't swing many votes.

In an interview last February Mr. Mondale did depart from his usual refusal to suggest he differed, silently, from some of the less popular decisions of the Carter administration. But otherwise he has taken the position that is both more appealing and, in the long run, the only politically viable position for one with his curriculum vitae: that on balance the president he served did a good job. Voters understand there are good reasons for a vice president to keep quiet about his disagreements with the president. And they know the important thing is what Mr. Mondale would do as president in 1985.

The Republicans can fairly argue that Mr. Mondale--or, for that matter, any of the Democrats-- would follow policies along the same general lines and with the same thrust as those of the Carter administration, and that such policies would be bad for the nation in the second half of the 1980s. But voters surely will reject as silly attempts to hold Mr. Mondale personally responsible for every jot and tittle of the Carter record.

Two things can usefully be said about Mr. Mondale's visit to Mr. Carter's vacation house in the mountains of north Georgia. First, both these public figures spoke plainly about what they were doing and why, without making any exaggerated claims. More than seven years ago Mr. Carter decided, after intensive study, that Mr. Mondale would be a more suitable national candidate than several rivals. He continues to think so.

Then, the endorsement is a reminder that these two men have made a useful organ out of what used to be the vermiform appendix of American government, the vice presidency. President Carter encouraged Vice President Mondale to become in effect a high-level presidential staffer--and therefore made him a valuable public spokesman and envoy. Ronald Reagan has followed a similar course with George Bush. It has not always been so: Dwight Eisenhower praised Richard Nixon only grudgingly in 1960, and Harry Truman endorsed Alben Barkley in 1952 "with nowhere else to turn," one biographer reports. But it seems only natural now that a former vice president running for president should get a wholehearted endorsement from his erstwhile chief.