Attorney General John N. Mitchell, in the heyday of the Nixon presidency, once memorably admonished critics of that administration's civil rights program to "watch what we do, not what we say."

It was good advice then, if double-edged, and it is even better advice now in assessing President Reagan's decision to renounce the SALT II arms control agreement while continuing in at least temporary compliance with its ceilings on nuclear missile launchers.

For ideologues on both sides, there was a curious cleansing quality in Reagan's statement that future U.S. decisions would be based on the Soviet military threat rather than on the requirements of SALT II. It satisfied conservatives who had waited 5 1/2 years for Reagan to make good on his 1980 campaign promise to renounce the "fatally flawed" treaty negotiated by President Jimmy Carter. And it ratified longstanding liberal fears that Reagan would prove a militaristic president who spurned negotiations with the Soviets.

Both sides seem more influenced by Reagan's words than his deeds, while being selective about the words they quote. In the same measured statement in which he ostensibly bid farewell to SALT II, Reagan said he would not deploy more strategic nuclear delivery vehicles or ballistic missiles than the Soviets. He also called upon Moscow to join in establishing an interim arms control framework of "truly mutual restraint."

While Reagan insisted he was dismantling two older Poseidon submarines for budgetary reasons, the effect of his action kept the United States within the limits of SALT II as the new Trident submarine Nevada began sea trials last week. Pentagon conservatives had wanted Reagan to dry-dock the Poseidons, rather than scrap them, pushing the United States over the SALT II ceiling of 1,200 nuclear delivery systems.

Instead, the United States will remain within SALT II limits until the 131st B52 bomber equipped with air-launched cruise missiles is put into service later this year, and could fall back under the limit a few months later if additional Poseidon subs are retired.

All in all, the Soviets have little reason to be unduly worried about Reagan's announcement. Superpowers are frightened by fast-flying intercontinental ballistic missiles that could launch a first strike, not by relatively slow-flying bombers that arrive hours later. This is why the Soviets fear deployment of the MX ICBM, which has first strike capability, and Reagan's vaunted Strategic Defense Initiative (SDI), which has ultimate potential to protect or prevent a first strike.

Because the president and Defense Secretary Caspar W. Weinberger booted the MX issue when they came into office, Congress has given the green light for deploying only 50 of these missiles. SDI is in trouble, too, and Reagan's anti-SALT II rhetoric may have stiffened Democratic resolve to cut the budget for the missile defense system.

But on balance, Reagan's decision was a victory for administration moderates. Weinberger backhandedly recognized this the day after the decision when he prematurely buried SALT II and claimed its ceilings could be exceeded by the missile-equipped B52s as early as August. Take all bets against this wishful forecast. Other officials say the better bet is that the United States will remain in compliance until year's end, by which time Reagan presumably will have held his second summit with Soviet leader Mikhail Gorbachev.

What Reagan did last week was to lay down a useful marker for this prospective summit. He noted repeated Soviet violations of the SALT II treaty, but said he would review his decision in the fall and "take into account" anything the Soviets have done in the interim. The president left himself running room to change his position if given any reason to do it.

Why should Reagan be faulted for insisting that the Soviets observe an unratified treaty he has respected for the past 5 1/2 years? And why should his call for "mutual restraint" be interpreted as an invitation to a bigger arms race? Reagan's words were carefully chosen to permit him to go either way. And his actions spoke even louder than what he said.

Reaganism of the Week: Speaking to the National Association of Manufacturers Thursday, the president said: "You know, it's said that if all the economists in the world were laid end to end, they still wouldn't reach a conclusion."