SOCIAL SECURITY offers endless opportunities for posturing. A couple of prominent Democrats briefly succumbed to the temptation to try it. The Senate minority leader, Robert C. Byrd of West Virginia, and the House majority leader, James C. Wright of Texas, said on Tuesday that if the Social Security cost-of-living increases are going to be postponed, the June tax cut ought to be similarly postponed--or perhaps repealed--for people in the upper brackets.

On Wednesday, Mr. Byrd and Mr. Wright hastily added that they were certainly not thinking of actual legislation. They had been misunderstood, they said. They were merely speculating on the subject of equity.

The postponement of the cost-of-living increase is part of an intricate and delicate compromise that already includes a tax increase for retired people with incomes above the average. Mr. Byrd and Mr. Wright both know perfectly well that there is no chance--none at all--of passing it if it gets entangled with other tax quarrels. An attack on the structure of the June tax cut would produce nothing but a counterattack by President Reagan and the congressional Republicans. It would be a fine fight, along comfortable and familiar ideological lines, with many opportunities for all the leaders to deliver their favorite speeches. But it would end in a stalemate next summer, with not quite enough money in the Social Security fund to pay those monthly checks.

Mr. Byrd's denunciation of the equitable and philosophical aspects of next June's tax cut is a bit late. The big drop in the top tax rates has already taken effect. It was part of the 1981 Reagan tax cut bill, and Mr. Byrd voted for it. Mr. Wright's position is at least consistent, since he voted against the 1981 bill. But it might be noted that Mr. Wright's position, if he were to pursue it, would undercut Speaker O'Neill, who has given his full support to the Social Security compromise.

That compromise was worked out, with great difficulty, by a commission in which both parties were ably represented. Its recommendations are fair and reasonable. If they are now turned into vehicles for personal advancement and partisan advantage, the chance of passing them into law willl be zero, and the trust funds will shortly run out of money. Mr. Byrd and Mr. Wright might want to ask themselves whether the 36 million people living on Social Security would regard that as enlightened and courageous political leadership.