Prime Minister Menachem Begin, apparently undiminished politically by his testimony on the Beirut massacre of Palestinian refugees, leaves for the United States Thursday for what is being portrayed here as a major showdown on the issue of Jewish settlements in the occupied West Bank.

The buildup began last week with the announcement by Deputy Prime Minister David Levy that five new settlements would soon be established in the occupied West Bank. The announcement provoked a sharp reaction from the Reagan administration in Washington, and since then the Israeli press has been filled with ominous predictions about Begin's meeting Nov. 19 with Reagan.

The Israeli concern was heightened this week by a briefing for Israeli reporters by an unnamed U.S. official, later identified by Israeli officials as Geoffrey Kemp, the senior Middle East specialist on the National Security Council.

Kemp reportedly predicted that the president will make a strong new call for a freeze on settlements when he meets with Begin at the White House. The presidential adviser was quoted as saying that there is an acute new awareness in Washington that the Begin government's ongoing settlements policy is "literally and figuratively changing the map," and that if it isn't slowed soon there will be little left to negotiate in the West Bank.

If the settlements issue is indeed at the top of the White House agenda, it is far different from what Begin has in mind for his visit to Washington, according to Israeli officials.

A senior official said today the Israelis expect the question of a settlements freeze to come up, but not to dominate the talks.

"If they have a showdown, what does that give you," the official said. "The administration knows that if it wants a freeze that is not the way to do it. A public debate with Begin over the issue will bring the opposite result."

While there may be sentiment among some administration officials to press Begin hard on the issue, the Isreali official went on, "I don't think Reagan wants to waste his time on polemics with Begin over settlements." Instead, the official said, the Israelis expect the talks in Washington to center on the negotiations for a withdrawal of foreign troops from Lebanon.

In that case, the American and Israeli agendas would nicely coincide, for it is the apparently stalled troop withdrawal negotiations that are uppermost in Begin's mind as he leaves for the United States, according to Israeli officials.

Foreign Minister Yitzhak Shamir said yesterday the talks could drag on for months. In the meantime, the Israeli Army has found itself on occasion in the dangerous position of attempting to separate warring Lebanese Christian and Druze factions, and living in the uncomfortably cold and rainy conditions of the Lebanese winter.

With this as the dreary backdrop, Begin is expected to appeal for new U.S. efforts to achieve Israel's main goals in the negotiations -- the withdrawal first of the remaining Palestinian guerrillas from Lebanon, and the establishment of a "security zone" along the Israeli border in southern Lebanon.

"What we would like is to renew the tacit understanding we had at the beginning of the war," when the administration acquiesced in the Israeli drive to destroy the Palestine Liberation Organization forces in southern Lebanon, the official said.

U.S. envoy Morris Draper met for two hours late today with Shamir and Defense Minister Ariel Sharon, and sounded more optimistic than have the Israeli officials of late. Draper said, "we're making progress" in getting the troop withdrawal negotiations started, adding that he was confident that "all the problems will be resolved." Draper declined to predict, however, when the talks may begin.

Reagan called for a freeze on settlements in September when he unveiled his Middle East peace plan. He said at the time that "more than any other action [a freeze] could create the confidence needed for wider participation" in the Camp David autonomy talks.

Begin rejected the Reagan peace initiative out of hand, and in the meantime Jewish settlement activity has continued unabated in the occupied West Bank and Gaza Strip. This week, for example, the Defense Ministry announced that four military camps south of Hebron are to be converted into civilian settlements, with two of them to be turned over to the nationalistic followers of Gush Emunim, which is in the forefront of the settlements movement.

The Tourism Ministry this week also announced an ambitious "master plan for the development of tourism in Judea and Samaria [the biblical names for the West Bank] including a projected increase of 500 hotel rooms in Jewish settlements in the territory.

The renewed American criticism of the settlements policy has been dismissed by Israeli officials as a likely futile attempt to entice Jordan's King Hussein to join the Camp David talks. They have also argued that calling for a settlements freeze amounts to telling Jews there are some places they are not permitted to live, a line that Begin is expected to invoke in several scheduled appearances before American-Jewish groups before his arrival in Washington for his meetings with the president and Secretary of State George P. Shultz.

As Begin prepared to leave for the United States, there was no apparent sign that the admissions he made yesterday before the state board of inquiry investigating the massacre of Palestinian refugees in West Beirut had seriously damaged his political standing.

In some quarters traditionally critical of the government, Begin was derided for not being in control of his own government. The prime minister told the commission he was not informed of the entry of Lebanese Christian militia units into the camps until after the fact, and that he did not learn of the massacre until after it was over.

"Ignorance was the main thrust of the prime minister's testimony at the commission of inquiry hearing," the English-language Jerusalem Post said in an editorial.

"But ignorance is not necessarily a virtue, especially in the case of a prime minister and most especially in time of war," the paper said.

Traditionally friendly newspapers such as Yediot Aharonoth praised Begin's testimony as "impressive" and other observers said Begin's willingness to be questioned in public and the fact the inquiry board did not ask to hold a closed session with him could strengthen his popular standing.