Two questionable political weapons have been unsheathed by President Carter for possible use against Israel's Prime Minister Menachem Begin if - no matter what he says - he diddles the president and the United Nations on the demand for a rapid pullouts of Israeli troops from Lebanon.

Yet, despite deadly serious purpose, the effectiveness of their weapon or of any other exercise of presidential power over Israel's prime minister is not yet certain. Indeed, the generally low state of U.S. influence, as contrasted with 21 years ago, when President Eisenhower successfully ordered Israel to withdraw from its first Sinai invasion, affects the way Israel looks at the United States as much as it affects any other country.

The first of Carter's weapons is straightforward: a law long on the books requiring a ban on any more U.S. military aid to any country that has used American weapons against a foreign nation, except for self-defense. Begin claims that the invasion and takeover of one-tenth of Lebanon was for "self-defense" - but it will be termed offensive by President Carter if Begin doesn't quickly act on the Security Council order to get out.

The second weapon is more ambiguous but - as with the first - it could pose problems for Begin: A spelling out in detail of the U.S. position for an overall Mideast settlement.

Tenaciously resisted by all previous Israeli governments as equivalent to a hated "imposed" settlement, such a spelling out of the American position would put Begin on the political defensive.

"Carter means business this time," one administration official told us. "He feels more strongly than the so-called Arabists [pro-Arab diplomats] in the State Department."

Inside the White House, Carter's top aides talked bravely about the fact that "we are not defenseless" as they awaited Begin's arrival. But in hard fact, the mood was a bit apprehensive. Some with a long memory recalled nostagically the way Eisenhower went on national television on Feb. 20, 1957, with his no-compromise demand that Israel withdraw from every inch of the Sinai it seized in 1956.

Stating that Israel "insists on firm guarantees" as a condition of withdrawal, Eisenhower said he would be "untrue" to his office if he endorsed "the proposition that a nation which invades another should be permitted to exact conditions for withdrawal."

The invasion of Lebanon, dictated by the same security concerns that led to that first invasion of the Sinai, has now taken Israeli troops to the Litani River. Some administration aides fear that Begin, a touch bargainer, will employ a series if stratagems to exploit the strong new military position his troops occupy, and his equally new and strong political base following the March 11 terrorist attack.

One strategem: an immediate, partial withdrawal to a line six miles north of the Israeli border. That was to be the original farthest point of the Israeli advance into Lebanon. By dramatizing a quick readiness to pull back that far-despite the risks of Arab guerrillas penetrating the area to be vacated - Begin would come across as accommodating, making it politically embarrassing for Carter to bring over pressure to go all the way immediately.

A far graver stratagem now feared by some Carter Mideast advisers would be a Begin offer for withdrawal from Lebanon, but linked to a demand that the president stop pressuring Israel to change its policy never to give up the West Bank.

That was the Carter administration's top target when the Begin visit was originally planned. Before the terrorist attack of March 11, the president had approved a confidential agenda strongly pressing Begin to agree on eventual withdrawal from the West Bank and on abandoning Jewish settlements in the Sinai as part of an overall Sinai withdrawal plan. That agenda proposed a no-holds-barred carter attack in the privacy of the Oval Office against Begin's obduracy on the West bank and the settlements.

The plan was to lay out with great severity U.S. demands for enough flexibility by Begin to bring Egyptian President Anwar Sadat back into the negotiating game. No commitments were to be asked of Begin during his visit; only that he take back Carter's urgent appeals to his Cabinet for a complete review of Israel's basic positions.

Now presidential advisers worry that if Begin yields anything, it will somehow be tied in to his newest conquest of southern Lebanon. Compared with Eisenhower's authoritative call for Israel to get off the Sinai 21 years ago, backed by his great prestige as president, Carter has little more than the threat of an arms cutoff that Congress might reject and the political leverage of an "imposed" U.S. Plan that Israel would surely reject.

Little wonder, then, that Carter and his aides long for a return of the presidential power and prestige of two decades ago.