Geneva by Christmas. That is what Secretary of State Cyrus Vance tells visitors who inquire about American objectives in the Mideast.

But a Geneva conference can only get under way if the Israeli government of Menachem Begin moderates its stance appreciably. Though Washington has begun to crank up pressure, many signs suggest that the strength and seriousness to go to the mat with Israel.

Geneva, of course, is a first step in meeting the President's commitment to a comprehensive settlement in the Mideast. As laid down by Carter, such a settlement would include a homeland for the Palestinian Arabs and a peace treaty between Israel and the Arab states along something like the borders obtaining before Israel's triumph in the Six-Day War of 1967.

But Prime Minister Begin is not ready to return to anything like the 1967 frontier. While prepared to give back territory to Egypt and Syria, he is not ready to yield the occupied lands west of the Jordan River to any foreign sovereignty - whether that of the Palestinian Arabs who populate the territory or of Jordan and Syria, which have claims to it.

To block foreign control, the Begin governments is busily planting Jewish settlements on the West Bank. And it absolutely refuses to deal with the Palestine Liberation Organization, which most Arab states regard as the legitimate spokesman for the Palestinian cause.

American pressure on the Begin government began a few weeks ago with statements by the President and the Secretary of State condemning the West Bank settlements. On Monday, after calling in Israel's ambassador, the State Department issued a declaration asserting that "Palestinians must be . . . at Geneva."

Pressures on Israel are certain to grow when the U.N. General Assembly opens its doors later this month and the Israeli and Arab foreign ministers come to Washington. The talk in some quarters of the department is of "a crunch within a matter of weeks, not months."

Brave words, but hard for anybody - and especially the Israelis - to take seriously. For the Begin government enters the coming period in an extremely strong position. Militarily the Israelis have commanding superiority. Politically Begin's standing at home and with American Jews was powerfully solidified by the warm reception he received from President Carter on his visit to Washington. Diplomatically he has a foreign minister, Moshe Dayan, who enjoys great personal prestige in this country and is a supreme tactician who is especially good at ducking efforts to put Israel on the spot.

The Arabs, by contrast, remain divided. The PLO, with the moderate leadership of Yasser Arafat in trouble, has refused the minimum concessions to Israel demanded by the United States as a ticket of admission to Geneva. While Jordan and Syria would not go to Geneva without the PLO, President Anwar Sadat of Egypt just might. Moreover, the Russians, who back the PLO, favor convening the Geneva conference, and then figuring out a role for the Palestinians.

As to the Carter administration, it has lost authority because of the Lance affair. It faces strong opposition in the Senate on ratification of the Panama Canal treaties. If pro-Israeli senators were to join the conservatives, Panama would be sunk and with it much of the rest of President's foreign policy.

Then there is the curious tone of the administration's approach to the Mideast. Carter unveiled his Palestinian homeland scheme in an off-the-cuff statement at a town meeting. "OK with me," was what the President said not long ago about a highly questionable formula for bringing the PLO to the Geneva conference. And the latest State Department shot at Israel was delivered on the eve of the Jewish New Year.

Set against the anguish and agony of the area, this tone is almost frivolous. It squares not at all with the kind of decisions required for a general settlement. Which suggests to me that the administration ought to lower its sights, and go for something far less than a comprehensive settlement.