President Carter's probable decision not to let his great new frield, oil tycoon Armand Hammer, continue selling phosphate fertilizer to the Soviet Union is being influenced less by his grand strategy to punish Moscow for Afghanistan than by relatively frivolous questions of election-year politics.

Indeed, the imminent billion-dollar phosphate decision is causing political contortions that have startled even hardened sophisticates inside the president's official family -- those who do not surprise easily.

The bottom line is a familiar one in this administration: even correct decisions on so crucial a matter as propping up Carter's response to the Soviet military takeover of Afghanistan are dictated by rules of politics at home, not rules of high policy abroad.

Exhibit No. 1 in the political turbulence surrounding the imminent phosphate decision is Vice President Walter F. Mondale, for years one of the most powerful voices inside his party advocating ever-weaker restraints on U.S.-Soviet trade.

Mondale has switched, fast and furious, since the farm revolt was triggered by the president's courageous decision to embargo 17 million tons of feed grains for the perpetually meat-shy Soviet Union. The reason: compelled to carry the president's banner and defend the embargo throughout his own Midwest base, Mondale has borne the bloody brunt of farmer fury. That fury would double if U.S.-Soviet phosphate trade continues, fertilizing Russian grain production.

Mondale's private word to Carter these days carries no burden of ambiguity:

cut off Hammer's phosphate shipments to the Soviet Union. To Defense Secretary Harold Brown and national security adviser Zbigniew Brzezinski, both covered with scars from trade battles lost against Mondale (plus the State and Commerce departments), the transformation of Mondale is perceived as a miracle.

In the mid-'70s, Minnesota's Sen. Mondale defended the trading interests of a major Minnesota computer firm called Control Data. Mondale blocked an overdue Senate amendment to give the Pentagon more discretion in barring exports of sophisticated U.S. technology to the Russians.

His sudden enthusiasm for cutting off sales by Hammer's Occidental Petroleum is matched by his Minnesota colleague -- Agriculture Secretary Bob Bergland -- for the same reason. Both have been singed by high-flying Republican presidential candidate George Bush, who publicly opposed the grain embargo. To one farm audience recently, Bush ridiculed a policy that embargos feed grains "at the same time that we approve shipments of fertilizer, phosphates and superphosphates to help the Soviet Union increase their grain production."

Farm Belt political weight, reflected by Mondale and Berland, when added to Brzezinski's new foreign-policy clout, is viewed by White House insiders as all but certain to carry the president along. That is true despite the high cards held by the rich and mysterious Dr. Armand Hammer, at age 81 an insatiable promoter of U.S.-Soviet trade. His ties to the Kremlin extend from intimacy with Lenin 60 years ago to effusive praise for Brezhnev today.

Hammer's political agility was shown by his jump from Richard Nixon (activities that led to his guilty plea to a Watergate-connected misdemeanor) to Jimmy Carter. At the height of political attacks on Carter's Panama Canal treaties, Hammer came to the president's rescue. Rising dramatically from his front-row seat at a Nov. 2, 1977, briefing ot a blue-ribbon committee backing the treaties, Hammer pledged $50,000 "on behalf of myself and my company" to finance a pro-ratification publicity campaign.

Hammer is described by insiders as too shrewd to try back-door White House lobbying with Hamilton Jordan, Carter's chief of staff, who became a Hammer enthusiast during their fight together for the canal treaties. Hammer's lobbying efforts to save his lucrative contract have been directed mainly at the Soviet Embassy in Washington. He has explained at length to Ambassador Anatoliy Dobrynin that if Moscow does not start pulling troops out of Afghanistan soon, U.S.-Soviet trade may not recover in this century. o

Such a stunning Soviet reversal is completely ruled out in the foreseeable future, ensuring a continuing U.S. trade clampdown that unquestionably weakens the Soviet Union. The Irony is that when Hammer's phosphates are included in the clampdown, it will be not to show Brezhnev a grand strategy to deal with aggression. It will only show political expediency in the Farm Belt.