There may be times when certain Republicans wish they'd never heard of a grain embargo. Much less campaigned against it.

Jimmy Carter's partial embargo on grain shipments to the Soviet Union, as punishment for the Russians' invasion of Afghanistan, is quickly becoming Ronald Reagan's grain embargo.

Despite campaign promises, despite the opposition of prominent GOP senators, despite a Cabinet-level review and a White House meeting last week, there is no sign that the embargo will be lifted any time soon.

More and more it appears, as Agriculture Secretary John R. Block suggested last week, that the embargo will continue at least until the Soviet Union offers some gesture of conciliation toward the United States.

Block, a strenuous critic of the sales ban and ultrasensitive to Farm Belt displeasure with it, raised another point that may further try the political patience of farmers intent on bolstering U.S. exports: the possibililty that the underlying 1975 bilateral grain agreement with the Russians, which the embargo only interrupted, will not be renewed.

"There is great concern on my part," Block told a House Appropriations subcommittee. "Without some change in the embargo situation, it would be difficult to negotiate a new agreement . . . At some point, the embargo has to come off, but it seems difficult to think we can have an agreement with the embargo still on."

Afterward, Block said the United States has had no contact with the Soviet Union on renegotiation of the 1975 agreement, which allowed Moscow to purchase American grains in an orderly fashion to prevent the disruption of U.S. domestic markets.

Sen. Walter (Dee) Huddleston (D-Ky.), ranking Democrat on the Senate Agriculture Committee, recently touched on another point that has been lost in the great debate over the merits of the embargo and whether it has hurt the Soviet Union as much as President Carter intended.

"We seem to think the Russians will want to renew the agreement . . . They have been able to make up most of their shortfall from other world sources," he said.

The Defense Intelligence Agency has estimated that the Russians had to spend an additional $1 billion last year to obtain the grain, used mostly in feeding livestock. Critics note, however, that the embargo cost the U.S. government about $3.4 billion; that was the price of taking over the grain that would otherwise have been sold to the Russians.

U.S Department of Agriculture analysts say the embargo, along with poor harvests, diminished Soviet meat production and slowed agricultural growth. U.S. farmers suffered some short-term losses after the embargo, but the drought and a growing world demand by year's end lifted grain prices well above their pre-embargo level. And despite the ban, U.S. grain exports last year exceeded 1979 figures.

American farmers greeted the 1975 trade agreement with enthusiasm, since it meant the opening of an important new market on a regularized basis with potential for expansion, as in fact occurred each year after the pact was signed.

Their enthusiasm was dampened after Soviet troops poured into Afghanistan in late December 1979. Within days, using national security powers given him by export law, Carter invoked a ban on the sale of any grain in excess of the 8 million metric tons the United States was committed to sell under the 1975 agreement.

That meant that about 17 million tons of grain were withheld from shipment. The U.S. government agreed to buy the grain to prevent market upheaval and price depression at home, but farmers were furious -- in part because Carter had allowed a continuation of other types of nonagricultural exports.

The ban, most observers agree, cost Carter dearly at the polls in the mid-American grain belt last November. The farmer's hope, of course, was that candidate Reagan would follow through with his promise and lift the embargo once he moved into the White House.

But it hasn't worked that way.

Urban members of Congress -- mostly Democrats -- have kept up steady pressure for continuation of the embargo. A resolution introduced by Rep. Peter Peyser (D-N.Y.), with 75 cosponsors, expressing congressional intent to maintain the ban is expected to win House committee approval in the next week or so.

"The embargo is not going to be lifted. I'm sure of that," Peyser said last week. "My sources at the State Department indicate that Secretary (Alexander M.) Haig doesn't think this is the time to life it . . . To lift the embargo would send signals that would confuse the world and give the Soviets the idea that we are not prepared to deal seriously with their aggressive policies."

Peyser and others, meanwhile, are savoring the delicious political ironies of the situation: here are some of the Republican Party's leading hard-liners on Soviet policy arguing for an end to the ban, even though there is evidence that the embargo has caused problems and extra cost for the Soviets.

"No question about their inconsistencies," Peyser said. "The message is clear at home, too. When agriculture gets involved in these issues, the hard-liners change their tune when the constituents start complaining."

Among the more prominent Senate conservatives who oppose the embargo are Jesse Helms (R-N.C.), chairman of the Agriculture Committee, Armed Services Chairman John Tower (R-Tex.) and Finance Chairman Bob Dole (R-Kan.). All happen to represent large farmer constituencies.

Helms said last week that he hoped Reagan would lift the embargo because he has a farm constituency "that is willing to work with him. His image is far more important in this country than any image abroad . . . I said at our meeting that he should lift the Jimmy Carter embargo and say he has a Reagan embargo in his hip pocket and that it will be applied all across the board if it is needed."

Other farm-state Republicans, ordinarily as hard-line as Helms toward the Soviets, tend to echo that view.

Peyser and friends are loving it. "It surely is a contradiction," he said.

Some of these same people supported draft registration as a way of showing the Russians our seriousness. To lift the grain embargo would be a perfectly rotten message for our young people as to how policy-making works in this country."