Washington — As Walter Mondale probes for a chink in Ronald Reagan's armor, the President is deftly positioning himself to try to deprive his Democratic rival of a key target in the campaign: the issue of war and peace.
Polls show that the Reagan administration's lack of progress in controlling nuclear arms and reducing the dangers of war is the primary foreign policy concern of Americans. But Reagan planners are hard at work building up the President's image as a peacemaker and devising policies that, apart from their diplomatic value, could help defuse the peace issue and give the President leverage when election day rolls around. Among these steps:
* The President will meet with Soviet Foreign Minister Andrei A. Gromyko, a Politburo member, on Sept. 28, following the UN General Assembly meeting. Such visits were traditional before the Reagan administration, but this will be Mr. Gromyko's first White House visit since 1978 and the first time President Reagan has met with such a high-level Soviet official.
* The administration has decided to back ratification of the UN convention against genocide. Adopted in 1948 as a response to Hitler's effort to exterminate the Jews, and ratified by 90 countries, the treaty has been supported by a succession of US presidents but opposed by conservative groups in Congress.
* Bilateral negotiations toward peace in Central America continue between representatives of the United States and Nicaragua. While the talks, begun unexpectedly on June 1, have not produced substantial agreement, they enable the President to counter the argument that he seeks a military rather than a diplomatic solution to conflicts in the region.
Also in the context of US-Soviet relations, the administration is looking to increase sales of grain to the USSR. American farmers face a huge grain surplus this year, and, as the President travels to the Midwest, he can point to the fact that his administration lifted the US grain embargo that President Carter imposed on the Soviet Union following the Soviet invasion of Afghanistan.
US officials say that in the coming harvest year the administration may again offer the Soviets the option to buy more than the maximum grain tonnage allowed under the current agreement, a move that would help the President politically.
Announcing his planned meeting with Gromyko, Mr. Reagan said at a brief news conference Tuesday that the two leaders would confer ''on a range of issues of international importance. ... One of my highest priorities is finding ways to reduce the level of arms and to improve our working relationship with the Soviet Union.''
Speculating on Soviet reasons for accepting a Gromyko-Reagan encounter, administration officials suggest the Soviet leadership probably reached the conclusion months ago that Reagan is likely to be elected and that therefore they might as well do business with him.
By last June, says a State Department official, the two sides had cleared the deck on minor issues, such as upgrading hot-line facilities. Then came the flurry of exchanges over the possibility of talks in Vienna on antisatellite and space weapons. These petered out, but, says the official, represented a ''healthy development that laid the groundwork for moving forward.''
Independent experts say they think the Soviet leadership is definitely weighing the American election. ''They didn't have to have Gromyko see Reagan,'' says William Maynes, editor of Foreign Policy magazine.
''They must have thought about what that meant. They can't ignore the American reality, and have to come to grips with it. They're hedging that Reagan will win, much as they dislike the possibility.''
While one meeting with Gromyko might not produce a breakthrough, it could bolster the President's contention that he is seeking ways to ''rid the world of this (nuclear) threat.''
Even while he woos the broad American public with a conciliatory stance, however, President Reagan is also covering his political right flank, conspicuously invoking the Yalta Agreement in his campaign rhetoric. He and his aides are telling audiences, especially ethnic groups, that the US will never accept an interpretation of the 1945 agreement that suggests consent for the division of Europe into spheres of influence.
''That agreement never gave (the Soviet Union) the power to dominate the countries of Eastern Europe and Poland as they have,'' Reagan told a Pennsylvania crowd of Polish-Americans this week.
Some diplomatic experts view such campaign rhetoric as dangerous, on grounds it stirs Soviet concerns about a Western effort to undo the post-World War II arrangements. State Department officials say that while such rhetoric may make the Russians resentful, the US does not have a ''rollback policy.''