Walter Mondale faces the same problem that confronted many recent Democratic presidential candidates: To win they have had to demonstrate not only that they were willing to negotiate with the Soviet Union, but also that they would stand firm against the Soviets if the need arose.
Polls have consistently shown over recent years that most Americans want to see the United States negotiate on arms control and other issues with the Soviets. But the same polls have shown that most Americans don't trust the Soviets. Most want a strong defense.
Mr. Mondale has stressed that he, too, wants an effective defense. He acknowledges that the Soviets are ''tough adversaries'' and asserts that no agreement with them can ever be based on trust. Agreements must be mutual and verifiable.
But in his campaign, Mondale has placed the main emphasis on the need to talk with the Soviets and on his contention that President Reagan has failed to negotiate seriously. (Mr. Reagan has argued that he is ready to talk at any time. He also reminds his audiences that it is the Soviet Union which withdrew its negotiators from the Geneva arms control talks).
Mondale has committed himself to a US-Soviet summit within six months of taking office and to annual meetings thereafter. He says he is convinced that if US-Soviet talks are not moving forward, US-Soviet relations don't merely stand still; like a bicycle, they fall down. Reagan says past arms agreements with the Soviets have been flawed. Mondale believes that they have been vital and that more are needed.
In two other key areas, Mondale clashes sharply with the President:
Southern Africa. Mondale has criticized Reagan for ''cozying up'' to South Africa's apartheid regime. Reagan would argue that a friendlier attitude toward South Africa has given the US more influence over events.
Central America. Mondale stresses a need for reform and negotiation in Central America rather than the use of military force. Where the Democrat differs most sharply with Reagan is over aid to the contra rebels - Reagan calls them freedom fighters - and what Mondale describes as an illegal war in Nicaragua. Mondale would cut the aid to the rebels.
The Democratic contender also says that he would sharply reduce the US military presence in Central America. But Reagan has already cut the size of military maneuvers in Honduras. The number of US military advisers in El Salvador is relatively small. On this point, as on a number of others, once Mondale took office, his differences with Reagan might turn out to be narrower than now seems to be the case. Regarding the Middle East, Mondale claims that Reagan's plan of Sept. 1, 1982, ''torpedoed'' the Camp David process. But that process seemed to be going nowhere. Reagan's plan actually brought the President more closely into line with the Carter-Mondale approach to resolving the Arab-Israeli conflict. Mondale says he would move the US Embassy to Jerusalem, but that might prove difficult once he was in office and had to take Arab concerns into account.
Mondale would cut the sale of sophisticated weapons to the Arabs. But once in office, President Carter changed his mind on that issue. So did Reagan.