Republican presidential nominee Ronald Reagan said today that freeing delegates to the Democratic National Convention to vote for the nominee of their choice would be unfair to voters who participated in the Democratic primaries.

Supporters of Sens. Edward M. Kennedy's presidential candidacy, and a scattering of other Democrats concerned about President Carter's electability, have joined in urging a rules change at the convention that would unbind the delegates. Reagan was asked about that proposal today as he left a four-hour closed briefing on foreign policy issues.

"It's easy enough to talk about freeing the convention," Reagan said, adding that those who do so "ignore one fact. These delegates were chosen in primaries. When you free the delegates what you are in effect saying is that the people who voted that way have lost their franchise."

Earlier, Eugene V. Rostow, chairman of the Committee on the President Danger, emerged from a briefing of Reagan to say that he and others had warned the candidate of the peril of the Soviet Union's increasing nuclear capability.

Rostow likened the present to the 1930s -- "People were paralyzed when a little bit of action would have prevented World War II." He said that the United States must match the Russians in nuclear and conventional forces and said it was particularly important that the United States have a second strike nuclear capacity "or our diplomacy has no punch."

The former undersecretary of state said that the question of U.S. "military superiority" called for in the Republican platform and denounced Monday by Defense Secretary Harold Brown was a "non-issue." He said that the president has used the term superiority" and has claimed incorrectly that U.S. forces have superiority over the Soviet Union's.

After the meeting, Reagan was non-committal when asked whether he thought that the president was doing everything he could to make the facts known about his brother Billy's dealings with Libya. However, when Reagan was asked whether the president should go to Capitol Hill and testify before an investigating committee, he replied: "If he's got something that would clarify things, yes."