Alarming indications are reaching Washington and European capitals that Soviet operatives may be undermining President Carter's Mideast peace plans even before the reconvening of the Geneva conference, a suspicion that has caused U.S. diplomats to put all Soviet actions in the Mideast under intense scrutiny.

U.S. officials carefully withhold charges of sabotage on grounds that hard evidence is not available - at least not yet. But top European diplomats trying to promote Carter's Arab-Israeli settlement plans are less restrained.

Thus, an official European source regarded as impeccably informed told us that a major break in the attitude of the Palestine Liberation Organization (PLO) toward Israel as a sovereign state was secretly vetoed by Soviet diplomats in Damascus.

According to the high-level European official, the executive committee of the PLO met in Damascus the first week in November and agreed on the following points:

To issue a statement adopting language taken directly from Carter's formal pronouncements about an Arab-Israeli settlement and from U.N. Resolution 242. The resolution (adopted after the 1967 Six Day War) calls on Israel to withdraw from Arab territories and the Arabs to make a real peace with a sovereign Israel.

The Damascus statement, while not specifically mentioning Resolution 242, would have served to closed the procedural gap between the PLO and the United States. In effect, it would have proclaimed the PLO's readiness to recognize Israel's status as a permanent state while saving a little face by not linking that recognition directly to Resolution 242.

But just before the PLO's statement was to be issued in Damascus, Soviet diplomats - according to our highly informed European official - sabotaged it. The change toward the United States never occured.

No statements of any kind were made. Only 10 days later, a high PLO official suddenly announced that the PLO would cooperate with a reconvened Geneva conference - but only if invited to do so by the United States and the Soviet Union under terms of the U.S. Soviet joint statement issued last Oct. 1.

That joint statement was attacked by Israel the moment it became public. Moreover, anti-Soviet hard-liners with [WORD ILLEGIBLE] special commitments to Israle were [WORD ILLEGIBLE] at its implications: bringing Moscow squarely back into the Middle East after almost a decade of official U.S. policy designed to keep the Soviets as far away as possible.

The PLO official who disclosed his organization's latest policy gambit was attending the Arab League foreign ministers' meeting in Tunis. His formula for PLO cooperation in a newly reconvened Geneva conference would drastically complicate U.S. diplomacy and aid the Soviet Union.

It would embarrass Carter by challenging him to get U.N. sanction for the joint U.S.-Soviet statement. He cannot disavow that joint statement because it is his own; but he cannot seek its adoption by the U.N. Security Council (as the PLO wants) without severe political reprisals here and in Israel.

Some well-informed U.S. officials question the depth of Soviet involvement in the PLO's unexpected switch from a strictly American policy in Damascus to a Soviet policy 10 days later in Tunis. Their point: The PLO is no more controllable than a herd of wild horses, by the Kremlin or anybody else.

Whatever its influence on the PLO, the Kremlin's enthusiasm for collaboration with Washington quickly faded after some early supporting statements. Soon after the joint U.S.-Soviet statement was made public, Moscow seemed to lose interest in backing the U.S. case for the Mideast settlement with anything like full vigor. Within the last two weeks, Soviet policy seems to have switched from simply losing interest to positive undermining of the U.S. position - as indicated by the switch in PLO strategy.

If the PLO had gone ahead as planned at Damascus, it would have paved the way for close U.S-PLO collaboration after months of Carter's efforts to achieve just that. It would also have raised Carter's prestige at a time he needs every ounce he can get.

That is why American officials, though unable to prove Soviet skull-duggery in the latest meandering of the PLO, have placed a double-watch over all Soviet Mideast moves. Self-congratulations for the joint U.S.-Soviet statement of Oct. 1 have turned into suspicions - and that is a healthy state of mind for the President still learning his way around the world.