WHEN SOVIET Foreign Minister Andrei Gromyko walked away from his meeting with Walter Mondale Thursday, the usually poker-faced Soviet nyet-man was smiling.

That smile said a number of things. It was, first, a calculated effort to demonstrate Soviet pleasure at the upcoming resumption of talks with leaders of the United States. It was also meant to cast Mondale as the more reasonable, moderate half of the Presidential candidate duo, as a practical statesman aware of the urgency of superpower communications. At a brief and formal meeting with the President earlier in the week, by contrast, Gromyko was careful to maintain an expression of utter solemnity.

"Some ideas suggested by [Mondale], should they materialize in Washington's policy, would upen up certain possibilities for bringing the positions of the two powers closer on arms limitations and disarmament," Tass, the Soviet press agency, said encouragingly.

But despite the hopeful smiles and the enthusiastic statements, the Soviets were exercising caution in voicing their preference for the 1984 election. Too obvious an endorsement of Mondale would almost certainly have backfired, prompting many voters to desert the Democratic nominee as too conciliatory and, in U.N. Ambassador Jeane Kirkpatrick's words, "soft on Communism." The risk of such a backlash, significant in any election, would have been compounded in this race by Reagan's remarkable ability to maintain a monopoly on flag and patriotism--and his repeated attempts to associate Mondale with the Carter Administration's perceived inability to adequately stand up to Soviet expansionism, most notably in Afghanistan.

The Soviet tacticians, it seems, have learned their lesson well. According to Richard Pipes, Baird Professor of History and a former member of President Reagan's National Security Council, the Russians have been backing candidates in Western elections to differing degrees since the early 16th century. Only in recent times, however, has their lobbying been carried to blatant extremes. On the eve of elections in West Germany last spring, they made it abundantly clear that Moscow was rooting for the Social Democratic Party, even going so far as to warn that if the Christian Democrats triumphed in Bonn, the result could be war. Such heavy-handed maneuvering was bound to have an adverse effect: scores of West German voters switched their allegiance to the Christian Democrats and the SDP, the ruling party, lost.

Suitably chastised, the Russians seem determined to exercise caution in their dealings with the volatile American electorate. While they did agree to meet with challenger Mondale, they avoided the sort of biased pronouncements that characterized their dealings in the spring.

Of course, there is yet another reason for the Soviet decision to publicly mince their words: as Assistant Professor of Government Mark R. Beissinger, who teaches a course on contemporary Soviet politics, points out, "as much as they publicly protest Reagan, the Soviets are not overjoyed either with Mondale." They too remember the days of Jimmy Carter, whose vigorous human rights campaign and unusual arms control maneuvers irked them. Mondale may be more willing to talk to the Russians than the current President, may support the cancellation of the controversial MX missile and B1 bomber and advocate a major down-scaling of defense expenditures--but he is by no means a pushover as far as the Soviets are concerned.

It was exactly Mondale's maintenance of a hard-line stance during the last week that prevented his early meeting with Gromyko from generating any harmful backlash. The Democratic candidate handled the sensitive situation with a deftness and caution which bespoke his past experience as a behind-the-scenes negotiator. He kept quiet about precisely what was said in his discussion with the Soviet Foreign Minister. He also carefully avoided jumping into actual negotiations himself, advising Gromyko in no uncertain terms to go back to the bargaining table with the Reagan Administration and upholding the President's supreme jurisdiction over national security and diplomatic matters. Despite the heatedness of the Presidential campaign now, Mondale last week did not try to steal Reagan's limelight--and in the process, emerged as a more responsible and mature leader himself.

It is unlikely, of course, that any major advancements in U.S.-Soviet relations will be made before the elections. The Reagan Administration would love to negotiate a headline agreement within the next month, a feat which would undoubtedly assure the President a coronation in November. The Soviets understandably are hedging their bets, stalling until they know exactly with whom they must deal for the next four years. They also have some major house-cleaning to do at home: as Pipes notes, "their largest problem right now is not our election, but a crisis of leadership in Moscow."

Yet however slim the chances for progress in negotiations between the superpowers may be in the near future, one comforting conclusion can be drawn from the historic Mondale-Gromyko meeting Thursday. Both participants approached the table with a soberness and professionalism befitting the major problems their countries must face--a welcome change from the bluster and ideological posturing that has emanated from both the White House and the Kremlin in the last few years.