A leading figure in West Germany's main opposition party said today that President Carter's policies and the personnel upheaval - in his administration are alarming West Europeans, cutting to the core of European security and "endangering the life" of the North Atlantic alliance.

"The belief in the American security guarantee for Europe does not only stand on power," said Christian Democrat member of parliament Alois Mertes, a party specialist on foreign and security policy. "It stands especially on the trust of Europeans in the political leadership, capability and calculability of the American president. The constant insecurities and uncertainties are, for that reason, endangering the life of NATO," he said.

Mertes, a frequent visitor to the United States, is well known among U.S. congressmen and has a reputation as a supporter of strong U.S.-West German ties. He said these concerns must be openly discussed and that he spoke as an act "of well-understood loyalty which was the opposite of anti-Americanism."

Mertes' views appeared in an interview on the front page of the respected Franfurter Allgemeine newspaper today and Mertes' office confirmed the accuracy of the report.

It was the first major public criticism of recent events in Washington by a leading West German politician. The Christian Democrats, although in opposition, are the largest single party in West Germany and form the nucleus of the conservative bloc.

In the current situation, Mertes said, one was forced to sound the alarm because "Soviet policy is coming to be more effective toward the European allies of America - because it was more disciplined, competent and calculable - than the one coming from Washington."

Mertes portrayed Soviet strategy toward Western Europe as skillful, switching from the tactic of confrontation to one of influence, which had as its long-term goal the estrangement of Europe from America.

Soviet policy, he said, "speculates on a slow worsening of the German trust in the dependability of the geographically distant and, for the first time, militarily vulnerable America and at the same time a discreet intimidation of the Europeans by enormous military superiority of neighboring Russian forces."

Mertes said he doubted that Carter and his White House chief of staff, Hamilton Jordan, recognized sufficiently what he called "this Russian masterpiece."

Mertes said he did not see any acceptable alternative for West German politics other than the alliance with Washington. Change could only come about if Moscow completely overturned its policy toward the entire German nation, meaning East and West, and under present conditions yielding to Moscow "would be suticidal" for West Germany.

In West Germany today, he said, trust in the United States "is still alive, but I know from many talks with people from all social levels that it is greatly endangered." Trust, he said, cannot be decided or come about by praying. It rather has to grow from inner conviction and must root in credibility.

In view of what he called the obvious weakness of American leadership, Mertes said it was necessary for both Chancellor Helmut Schmidt and opposition leaders to balance more closely their foreign policy views. Schmidt, and opposition challenger Franz Josef Strauss, have one thing in common, Mertes said: a "great concern about America."

Mertes' comments came one day after Chancellor Schmidt returned from a private vacation in the United States that, nevertheless, turned out to be politically interesting. Schmidt was invited to California to visit his long-time friend George Schultz, who was Treasury Secretary under president Nixon at the time Schmidt was finance minister in Bonn.

The trip was arranged before the Carter Cabinet upheaval.

While in California, Schmidt was Shultz' guest at an elite club of industrialists and diplomats and had discussions with former secretary of state Henry Kissinger and former NATO commander Gen. Alexander Haig.

In 1976, Schmidt let it be known that he favored President Ford over the Challenger, Jimmy Carter, primarily on the ground that Ford was a known quantity in Bonn.

West German officials are playing down Schmidt's meetings in California. They tend to emphasize that the chancellor also met with Carter's nominee to head the Treasury, William Miller, and had an airport meeting in Washington with National Security Affairs Adviser Zbigniew Brzezinski and a telephone conversation from the airport with the President Carter.

No substantive details of Schmidt's California discusions have been revealed. CAPTION: Picture, At his news conference Wednesday night in the ornate East Room of the White House, President Carter picks a questioner among the flock of reporters. By John McDonnell - The Washington Post