THE ADMINISTRATION'S decision to vote to seat Pol Pot in the United Nations is an abysmal exercise of hold-your-nose diplomacy. This year as last, the State Department insists that it abhors the Pol Pot record of genocide in "Democratic Kampuchea" (Cambodia). It says its vote does not imply recognition of a detested unrepresentative regime but merely a calculated decision to stand with its Southeast Asian friends in opposition to the puppet regime that invading Vietnamese installed in Pol Pot's place. Given the administration's sure fore-knowledge that a Pol Pot vote would be taken -- fairly, we might add -- as a mockery of its devotion to human rights, there is no reason to doubt that it believes what it says.

But this is not, we think, another instance in which Jimmy Carter has put moralism or softheadedness reluctantly aside in favor of a latter-day conversion to realpolitik. For the decision is also questionable in realpolitik terms. What abiding interest of the United States is served by according a renewed measure of aid an comfort to Pol Pot, a man defended by his warmest foreign admirer, China's Deng Xiao-ping, on grounds that Pol Pot made "serious mistakes" bud did not really kill more than a million Cambodians? In asking the American people to support a policy that brings smiles and thank-yous from a gang of mass murderers, Mr. Carter has made a serious political error. People will not support such a policy. They should not be asked to. There are many close calls in foreign policy, but his is not one of them.

But, it is asserted, the United States must stand with its Asian friends and allies -- China, Thailand and others -- in resisting legitimation of a regime installed by the agression of a Soviet pawn state, Vietnam. In fact, the Asian countries in question are not so much registering a principled opposition to aggression as they are playing in their separate ways an old Asian power-balancing game -- a game, moreover, that many of them will play differently as events move along. Since Vietnam's invasion of Cambodia in 1979, the United States has had many other ways available to it of registering opposition. It did not need to vote for Pol Pot. It could have voted to keep the seat vacant in order to assert the central and miserable truth of the matter, which is that no regime claiming legitimacy in Cambodia has earned it. The United States owes Cambodians no less.