Barely able to keep alive his once bright hopes to bring peace to the Mideast, President Carter has now been reduced to a series of intricate strategems that seem preposterous for normal diplomacy.

The major reason for this diplomacy-by-strategem is the warning from powerful members of Congress of both parties that public pressure on Israel by Carter would be self-defeating if not politically suicidal.

These warnings have been conveyed privately to the President, effectively foreclosing the most potent weapon the United States has in its day-to-day diplomacy: presidential pressure, such as publicly dramatizing major differences between U.S. and Israeli interests in the Mideast.

The most important strategem now under private consideration - essential if the President's peace plan is to survive the next few weeks - is to invent some way to bring the Palestine Liberation Organization (PLO) into the peace game despite Israel's Geneva conference veto.

Under consideration is an unconventional plan to bypass the Geneva conference altogether and switch the action somewhere else - perhpas as an added attraction for this fall's session of the United Nations General Assembly.

The reason for this switch: a 1975 commitment given in secret (but soon leaked to the press) by then-Secretary of State Henry Kissinger. Kissinger pledged to Israel an absolute right of veto over PLO attendance at Geneva.

Since overt White House pressure against Israeli Prime Minister Menachem Begin to nullify this pledge would boomerang, Carter may be forced to move the Arab-Israeli peace conference in a strategem designed to get around an Israeli veto.

But first Carter must be able to justify the PLO's bid to attend a peace conference as representatives of the one million West Bank and Gaza Arabs now living under Israeli military occupation. For that, the President has to gain the PLO's acceptance of Israel as a permanent state.

That leads to the second diplomatic strategem: a subtle change in wording that Carter used for the first time last week to nudge the PLO into accepting Israel as a state.

In the past, the United States has said that "unless" the PLO endorsed United Nations Resolution 242 (thereby signaling acceptance of Israel) the United States would not even consider dealing with it or sponsoring its presence at peace talks.

But without public announcement Carter subtly changed that formulation last week. He now say that "if" the PLO accepts resolution 242 the United States will "immediately" open direct talks with it. That would lead to U.S. support of the PLO at peace talks with Syria, Egypt, Jordan and Israel.

Carter's switch from "unless" to the more positive "if" was calculated for its impact on Saudi Arabia, back-stage power broker in the Mideast peace game. The President, both in his Time magazine interview last week and in a conversation with the press in Plains, thus sent the Saudis this signal: If they would intensify the effort by moderate Arab governments to persuade the PLO to accept Resolution 242, the United States would promptly start dealing directly with the PLO and bring it into peace negotiations.

That signal was quickly seized by the Saudis, who informed Secretary of State Cyrus Vance when he arrived in Saudi Arabia that the PLO was close to a possible change.

That the mighty United States must resort to such strategems tells much about its political weakness when trying to turn the tables on Israel.

Discovery of this fact may be President Carter's principal accomplishment so far in trying desperately to write a fair formula that would end the cycle of war in the Middle East. He started with a grand design and courageously built his case to climax at a reconvened Geneva conference in October.

He has now been brought hard to earth by the Israeli reality and is reduced to subtle strategems and murky machinations.