Shrill critcism by respected liberals of President Reagan's foreign policy, aimed foremost at U.S.-Soviet relations, is playing into the hands of growing anti-American tendencies in Western Europe and giving the Kremlin new stature.

Indeed, it is arming the Kremlin's propaganda machine with powerful ammunition in the battle to exploit European fears about NATO's nuclear modernization program.

Reagan has been postponing a major foreign policy speech on one occasion after another, but the intensity of the attack by leading figures of the flawed, once-potent Eastern foreign policy establishment, now leaves him no alternative. Political and diplomatic advisers believe the great communicator can easily explain the principles that guide a foreign policy his liberal critics find unacceptable, but that he must do so soon.

What triggered the mood of urgency was shock from the president on down at former secretary of state Cyrus Vance's surprising charge on "Meet the Press" June 21. "It is a real question in Moscow," said Vance, "as to whether or not there is any possibility of resuming serious negotiations" between Washington and Moscow.

Although Vance's sincerity is not questioned, that mirrors propaganda coming out of the Kremlin, led by President Leonid Brezhnev himself, to undermine the credibility of the Reagan administration. "Vance is now telling our European friends and his country that Brezhnev is right and Regan and [Secretary of State] Al Haig are wrong," one incensed official told us.

The recent string of embarrassments suffered by Haig and the White House on peripheral issues puts a thin gloss of credibility on the attempt to savage the Reagan-Haig performance. The unseemly assault by Haig's aides on United Nations Ambassador Jeane Kirkpatrick damaged Haig. Likewise, the clumsy White House announcement on July 1 that those F16 aircraft withheld from Israel would -- or would not -- be released portrayed confusion.

But that is gloss. Vance and other critics are striking at the heart of the Reagan-Haig policy and pronouncing judgments seemingly based on this criterion: is the policy acceptable to the Russians? Asked whether Reagan's decision to make Communist China eligible for lethal arms would encourage Soviet intervention in Poland, Vance said the arms decision "reduces any leverage we may have with the Soviet Union."

Likewise, probed about the administration's long-planned refusal to be rushed into strategic arms negotiations with Moscow, Vance replied: "I do not believe that they [the Reagan administration] have a policy with respect to arms control." From a former secretary of state whose first arms control initiative for Jimmy Carter in 1977 was a bust and whose second attempt resulted in a treaty the Senate refused to ratify, that response had Orwellian overtones.

Much of the Reagan-Haig record in handling the Soviet Union remains out of public sight. Top officials, including U.S. diplomats in Moscow, have engaged in more than 50 private talks with Soviet officials, many not publicized. More to the point, during the presidential campaign Reagan carefully differentiated his arms control thinking from the Nixon-Ford-Carter failures, removing SALT from its elevated shrine.

Top to bottom, Reagan's advisers are determined not to be sucked into a SALT-directed Soviet policy of the kind that now leaves the United States dangerously exposed to Soviet strategic power. They will not dilute their insistence that Soviet conduct itself will determine American policy.

Vance's chastisement of the president over his decision to make China eligible for lethal weapons was particularly galling to Reagan operatives (and has been publicly countered by Zbigniew Brzezinski). Vance fought prevailing Carter administration policy to pull the United States and Communist China closer. Vance and his top Soviet adviser, Marshall Shulman, argued it would be "provocative" to Moscow.

Vance even resisted Carter's decision to give China most-favored-nation treatment -- because the Russians did not have it. It was an open secret that if Carter had been re-elected, the ban on sale of lethal arms to China would have been lifted.

Soviet propagandists are making such skillful use of the liberal campaign against the Reagan-Haig foreign policy that Afghanistan is becoming a forgotten issue. Some presidential advisers think an answer is overdue and that Reagan must make it soon.