Vice President Mondale today reiterated the Carter administration's belief that direct negotiations at a new Geneva conference are "crucial" to a Mideast settlement.

In the first comprehensive restatement of American policy since Menachem Begin's victory in the Israeli election, Mondale signaled a new effort by the Carter administration to press both Israel and the Arabs for an overrall Middle Eastern settlement.

He told reporters en route here from Washington that Carter would invite Begin to meet with him in Washington as soon as Begin's minority Likus Party has completed its formal assumption of power.

Begin's past militance against any terroritorial concessions is potential roadblock to negotiations, but Mondale said that while reports on Begin's present attitude differ, "we are taking a hopeful assumption." He said Secretary of State Cyrus R. Vance is planning another Middle East mission, possibly in late July, after the Carter-Begin talks.

If everything works well, he said, Geneva talks might be under way by late autumn.

In a 35-minute address to a meeting of the World Affairs Council of Northern California, the Vice President appeared to be seeking to reassure American Jewish leaders, who have expressed fears since Begin's victory that the United States might put extreme pressure on the new Israeli government to counteract Begin's stated determination to retain the West Bank territory won in the 1967 war.

While breaking no new gound in policy terms, Mondale emphasized several points of concern to supporters of Israel in Congress.

He said, "We do not intend to use our military aid as pressure on Israel . . . We will not alter our commitment to Israel's military security."

He also said that America believes Israel "should not be asked to withdraw (from occupied territories) unless it can secure in return real peace from its neighbors." And he reiterated Carter's view that "the first prerequisite of a lasting peace" is the recognition by the Arab countries and the Palestinians of "Israel's right to exist permanently . . . and its right to exist in peace."

At the same time, Mondale emphasized Carter's previously stated view that the permanent borders of Israel should be "approximately the borders that existed prior to the war of 1967, albeit with minor modifications as negotiated among the parties.

He also underlined the proposal - previously advanced by the President - for "security lines" outside those permanent borders "that would ensure Israel's safety as full confidence developed in a comprehensive peace."

As the third element of the proposed comprehensive package, Mondale said the administration supports "a Palestinian homeland or entity - preferably in association with Jordan."

After outlining the elements of a possible agreement, Mondale conceded that "this is obviously a difficult task and there is always the possibility of failure." He said direct talks in Geneva were essential to test the possibility of peace this year.

"We cannot conceive of genuine peace existing between countries who will not talk to one another," he said. "If they are prepared for peace, the first proof is a willingness to negotiate their differences."

The speech Mondale gave here was largely written by the National Security Council Staff and was added to a weekend schedule of political appearances in California, Arizona and Missouri.