Republican presidential candidate Ronald Reagan said today that the United States and its allied should suspend trade with the Soviet Union until Russian troops are withdrawn from Afghanistan.

"Why shouldn't the Western World quarantine the Soviet Union until they decide to behave as a civilized nation should?" he said at a news conference here.

When a reporter asked Reagan whether he favored "a total cutoff of all trade with the Soviet Union," the former California governor replied: "Why not" It sure beats a war."

Later, in response to question at a news conference in Burlington, Vt., Reagan said such an option should be "considered" and that he was not yet prepared to recommend it. He added that any trade cutoff should be made in concert with U.S. allies.

Reagan last week opposed President Carter's imposition of a feed grain embargo against the Soviets, saying that the action would penalize the American farmer while leaving the Russians free to buy grain elsewhere. Since then, however, other major grain-producing nations have agreed to support the U.S. embargo.

On a busy day of campaigning in New England, Reagan all but abandoned his basic set speech in favor of question-and-answer sessions and free-for-all news conferences which plunged him into various controversies.

In Bedford, Reagan said he suspected that the Iranian expulsion of U.S. reporters was the prelude of Soviet thrust into Iran and seizure of the oil fields there.

Reagan said he had reached his conclusion by "using a little logic." The Soviet Union, he said, had built "a network of roads that could be used for the invasion of Iran."

Last month, Reagan and other presidential candidates were offered a briefing on the Iranian situation by Secretary of State Cyrus Vance. Reagan said Vance's invitation "was so late and so much had been done" that he never accepted it.

Afterward, Reagan's press secretary, Jim Lake, said that the invitation had been rejected because "so much had been put out by the administration in the media about Iran that we thought the only purpose . . . a briefing could serve would be to co-opt us."

Reagan's opposition to the Equal Rights Amendment came up today on three occasions, once during a forum with the student body of the Goffstown High School, again at the Bedford question-an-answer session and a third time at the Manchester news conference.

On each occasion Reagan said he favored legislation against sexual discrimination rather than a constitutional amendment for the purpose. He said passage of the ERA would open "a Pandora's box" in which legal protections of women, such as a father's responsibility for child support, could be challenged in the courts.

Reagan said he had no objection to a woman as his running mate, but added that polls taken by his campaign in 1976 showed that a majority of people opposed the nomination of a female vice president.