Things are so bad on arms control that the White House would rather talk about Social Security, which used to be the most aggravating issue on the president's desk.

"Oh," said press spokesman Larry Speakes, with genuine regret, when someone at the press briefing asked about Eugene V. Rostow, the hawkish director of the Arms Control and Disarmament Agency whom the president shot down last week. "You want to switch off the Social Security issue?"

He sighed and began reading a prepared statement, to put the whole thing "in perspective."

Rostow, as he angrily packs up the World War I posters that hung on the ACDA walls has been singing to the press. His lament: he got fired for trying to advance the cause of world peace.

Specifically, he said he authorized Paul Nitze, the negotiator for intermediate range missiles, to explore with his Soviet counterpart in private a plan for reducing the number of those missiles.

The plan subsequently was rejected by the Soviets, Speakes unhappily revealed. And it also was rejected by us, because as White House foreign policy press aide Mort Allin alertly explained, "it gave the Russians a monopoly and an advantage."

Why, you may well ask, would Nitze, who entertained such seditious nonsense, remain in possession of "the president's full confidence" while his superior has been dealt the ace of spades?

The State Department explains the matter starkly: "We would have been ruined in Europe if they both went. It would have confirmed that talking to the Soviets is fatal."

Nitze, it seems, is still empowered, despite the rebuke administered to him the first time he tried it, to explore possibilities with the Soviets and to look for flexibility in their position. The graver question remains: is there any flexibility in Reagan's position, or is it the "zero option" or nothing?

The White House puts out the word that all that has happened is that arms control now is under new management. Secretary of State George P. Shultz has taken charge. Why it took him so long to get around to what the president called last Friday "the most important problem facing this generation" is not clear.

The administration is working hard to render Rostow a non-person. He was difficult, it is said. Did not know his place. Pestered the president on subjects that were none of his business. It was not, as he suggests, "too much zeal" for arms control that led to his cashiering.

Speakes was asked if anyone in the White House was concerned about "making Rostow a dove."

"He is not in that job any more," he said, as if dismissal somehow had robbed the 70-year-old Yale law professor of his identity. "It is not a valid perception."

Certainly the liberal community is bemused to find itself mourning Rostow's passing. It was noted with melancholy at a nuclear-freeze conference that was held at Harvard last weekend.

"Imagine," said one of the academic participants who had done bitter battle with Rostow and his brother, Walt, who was President Johnson's national security affairs adviser during the Vietnam years. "Me, nervous because Gene Rostow is gone."

Rostow was favorably marked by Reagan as one of the heavy hitters of the Committee on the Present Danger, an organization of right-wing Democrats, which led the opposition to SALT II, a treaty Rostow called "an act of appeasement."

His designation as arms-control director was seen as a sure sign that not a leaf would fall in Geneva until the arms buildup was far advanced. The selection of Nitze was the nail in the coffin of peaceniks' hopes.

Nitze, whose hard-line credentials are crushing--he urged Americans to regard nuclear war as "thinkable"--apparently got caught up in what he thought he was supposed to be doing--working for an agreement, even if it was not part of the president's zero-chance "zero option."

At the Harvard conference, where the old arms control community and the new nuclear freeze movement had an unexpectedly friendly encounter, the most attended speaker was pollster Louis Harris, who told of the "counterforce" that Reagan has created on arms control. By a margin of 66 to 31, Americans rate as "unsatisfactory" the job Reagan is doing in arms negotiations.

By 57 to 39, they say they are worried that Reagan "may get the country into a major nuclear exchange." At the same time, 51 percent say they believe the Soviets are an "outright enemy," and an overwhelming 85 percent say they believe that the Soviet Union is a "power hostile to the U.S."

And still, they favor, by 76 to 21, a nuclear freeze.

And that is why anyone in the White House would rather talk about increased taxes and reduced benefits in Social Security than the ins and outs of arms control.