Former senator George McGovern said yesterday that he would advocate a 20 to 25 percent cut in defense spending and a unilateral halt to further U.S. nuclear weapons development in his long-shot bid for the Democratic presidential nomination.

McGovern told a group of reporters that the military cutback and the moratorium on new nuclear weapons could be put into effect without negotiation with the Soviet Union and without risk to American security.

He offered the two positions as examples of policy stands he will take that will differentiate him from the other six Democratic contenders, all of whom, he said, favor increases in Pentagon spending. McGovern said he would begin his active campaigning this week with two appearances in Maine and one in New York state, then will concentrate heavily in October in Iowa and New Hampshire, the first caucus and primary states.

Saying he had received $30,000 in contributions so far, the 1972 Democratic nominee said he expected to qualify for federal matching funds in the next 30 days and have a full-scale field organization in place by the end of the year. He said his goal was to finish "in the top four" in the first contests and conceded that he would have to establish his credentials as a winner by April 1 "or get out of the race."

Showing that he's not afraid to take the kind of positions that made him controversial in 1972, McGovern said he was disappointed that Hill Democrats and other presidential hopefuls haven't challenged President Reagan's contention that U.S. military spending must increase to deal with the Soviet threat. "We ought to be cutting defense 20 to 25 percent, rather than increasing it," McGovern said, adding that such reductions could be achieved "just by eliminating wasteful spending."

"If I were president . . . ," McGovern said, "I would not request any additional nuclear weapons at all. We don't need bargaining chips . . . . The truth is we're leading the arms race . . . . The time has come to quit building these things--no matter what they the Soviets do."

Saying he could not think of one weapon the Soviets have introduced first, McGovern said that if the Soviets continued to build nuclear weapons after the United States stopped, "I would just say they are more stupid than I thought."