Richard M. Nixon is on the loose again these days, freely offering cogent advice on how the United States should negotiate arms-control agreements with the Soviet Union. It is interesting to speculate on what might happen at the Geneva summit if President Reagan decides to listen to him.

Meeting with journalists last month, Reagan said he consults frequently with Nixon, whom he praised as "most knowledgeable on international affairs." Nixon displayed this knowledge in two recent magazine articles that take a hard-eyed view of Soviet intentions while critically examining Reagan's much-ballyhooed Strategic Defense Initiative, frequently called "Star Wars."

Nixon is neither soft on the Soviets nor sanguine about arms control. He contends in the conservative biweekly National Review that the Soviets seek "victory without war" while the U.S. goal is "peace without victory," which Nixon says is "potentially a recipe for defeat for the West."

In the journal Foreign Affairs, Nixon argues that "it is a mistake to support arms control as desirable in itself and to believe that any agreement is better than none." He says that "a bad agreement that opens the way to Soviet superiority increases the danger of war." These warnings should resonate with the president, who is usually more aware of the dangers of arms control than of its benefits.

But Nixon is not reassuring to those who share Reagan's extravagant vision that missile defense is "the essence of science and spirit joining for mankind's highest ideal -- peace on earth."

Nixon agrees that the Strategic Defense Initiative (SDI) is indispensable to arms control because of the incentive it has provided to the Soviets to propose a limit on offensive-weapons systems. However, he dismisses Reagan's notion of SDI as a heavenly umbrella that would protect civilians from nuclear destruction. The former president points out that even the most enthusiastic supporters of SDI agree that total population defense would not be possible well into the next century. "This is small comfort to those who will live and die in this century," Nixon observes.

Nixon makes a distinction that the Reagan administration has ducked between a globe-spanning plan and a more limited system that would defend missile sites. He also cautiously endorses a third option, a "thin population defense" that could be used against an accidental launch or an attack by a minor nuclear power.

But a defensive system that protects U.S. missile sites and preserves U.S. retaliatory capability is what Nixon finds most promising. He maintains that deployment for defense of U.S. missile fields is "the ultimate bargaining chip" and says the United States should agree to limit deployment of such a system only if the Soviets agree to limit their offensive weapons.

It would be refreshing to hear such realism from Reagan, who instead told Republican leaders last week that SDI was "not a bargaining chip" and gushed that it would "demilitarize the arsenals of Earth." Later, one GOP congressional leader privately complained that Reagan had talked himself into believing that SDI was "the Second Coming." The confusion continued the next day when a senior administration official told reporters that "defense could be lessened to the degree that offense was lessened," which sounds more like what Nixon was suggesting than Reagan's vision.

If SDI is to fulfill its promise as a negotiating tool when Reagan meets Soviet leader Mikhail Gorbachev at Geneva next month, the administration ought to produce at least a unified view of what it is promoting. It would be something of a breakthrough if the two sides could agree on even a definition of terms, so that we know what Gorbachev means when he acknowledges the need for "fundamental research" on strategic defense and what Reagan means when he calls for "research and testing."

The vagueness and overstatement on the U.S. side may be partly a negotiating ploy. But, in the long run, missile defense would have a better chance of support from Congress and the country if Reagan took a less grandiose view of his proposal. He could start by reading Richard Nixon.

Reaganism of the Week: Welcoming Singapore Prime Minister Lee Kuan Yew to the White House last Tuesday, Reagan said, "Well, it gives me great pleasure to welcome Prime Minister Lee Kuan Yew and Mrs. Yew to Singapore -- Mrs. Lee to Singapore."