Ronald Reagan joined his major Republican presidential rivals today in criticizing President Carter's imposition of a partial grain embargo on the Soviet Union as retaliation for its invasion of Afghanistan. But the front-running GOP candidate repeatedly declined to say what he would offer as an alternative.

"I just don't believe the farmer should be made to pay a special price for our diplomacy, and I'm opposed to what's been done," Reagan said at a brief news confernce in a crowded hotel lobby here.

A month ago, in Santa Ana, Calif., Reagan suggested that the United States should embargo all trade, including foodstuffs, with Iran as retaliation for the Nov. 4 seizure of Americans at the U.S. Embassy in Tehran.

At a rally attended by 1,500 to 2,000 Republicans here tonight, Reagan for the first time rapped Carter for his actions aimed at securing the hostages' release.

"Anything that has been done by this administration so far could and should have been done starting in those first hours when our embassy was taken over and those people were taken captive," Reagan said.

The former California governor declined to say what further steps he would take as president.And in his earlier news conference, while sharply critical of Carter for failing to anticipate the Soviet invasion of Afghanistan, Reagan declined to say what he would have done or what he would do now to counter the Soviets.

Reagan refused to meet the Republican opponents at a debate in Des Moines last Saturday organized by the Des Moines Register and Tribune. He repeated today that he thought such debates were "divisive" for the GOP and said he did not intend to participate in them.

But he reacted tartly when a reporter quoted rival John B. Connally as saying in the debate that he doesn't know where Reagan stands on the issues.

"He must have been living under a rock," Reagan said. ". . . I've been saying what I believe in for a long, long time, and I spent eight years as governor trying to implement what I believe. I haven't changed those views, and I have been speaking in specifics."

Reagan declined, however, to go into specifics tonight. When pressed for the differences between himself and his opponents, he responded that "I want the nomination."

Reagan came to snowy Iowa from sunny Califonia after three weeks of noncampaigning. He has campaigned only 13 days since announcing his candidacy on Nov. 13, but he plans a stepped-up schedule now that the early caucuses and primaries are nearing.

Throughout the early campaigning, Reagan has maintained a wide lead over the other GOP candidates in various public opinion polls, and his Republican rivals are now beginning to concentrate their fire on the front-runner. In the Iowa debate last Saturday, several of the GOP candidates jabbed at Reagan for refusing to participate. A few made cracks about his age, which will be 69 on Feb. 6.

However, the man who is generally regarded as Reagan's most potent adversary in Iowa -- George Bush -- conspicuously made no criticism of the former California governor. A high-ranking Bush aide said afterward that this was a conscious strategy reflecting the belief that Reagan is liked or respected by Republicans of most persuasions.