The war in Lebanon has boosted Prime Minister Menachem Begin's ruling Likud Bloc to a historic level of popular support, a public opinion poll published in the Jerusalem Post showed today.

The poll, conducted between Aug. 10 and 19 by the Modi'in Ezrachi Research Institute in Tel Aviv, said that if elections were held now the party apparatus headed by Begin would capture 66 seats in the Israeli Knesset (parliament), an absolute majority in the 120-member body that no political party in Israel's history has ever enjoyed.

The poll results reflected a remarkable reversal in Begin's political fortunes as a result of the war and the military defeat of the Palestine Liberation Organization forces in Lebanon. A similar poll conducted in May, one month before the Israeli invasion of Lebanon, gave the Likud Bloc 51 Knesset seats, only five more than the opposition Labor Party. Those findings were a part of a continuing decline in the signs of public support for the Begin government.

In the latest poll, Labor Party support dwindled to 35 Knesset seats, a gap of 31 seats behind the Likud, which, according to the poll, today enjoys the highest level of public support in its history.

The stark trends in such poll findings show that while the war in Lebanon has produced widespread criticism of Israel in many parts of the world, back home, which is what counts to Israeli politicians, it is an entirely different story. The findings also suggest that the Labor Party, which is attempting to launch a nationwide debate on the conduct of the war, may be further isolating itself and seizing on the wrong issue in its effort to bring down the Begin government.

There is no question from the poll results that it was the decision to drive the PLO out of southern Lebanon that so dramatically boosted Begin's political fortunes. Asked whether now, knowing the cost and outcome of the fighting in Lebanon, they would support the government's decision to launch the June 6 invasion, more than 80 percent of those polled said they would.

In itself, this pro-invasion sentiment is not surprising. The Labor Party supported the invasion at the time, which the government announced was intended to clear the PLO from a 25-mile-wide corridor along Lebanon's frontier with Israel.

But the Israeli Army did not stop after 25 miles. Instead, it continued north to the gates of Beirut in what became the longest war in Israel's history except for the 1948-49 war of independence.

It is this decision specifically that Labor Party leader Shimon Peres has demanded be investigated by a national commission.

Today's poll findings suggest that the Labor Party cannot count on strong public backing in pressing this demand. According to the results, 64 percent of the Israeli public supports the decision to go beyond the initial 25-mile target, including 46 percent who agree that it was the right thing to pursue the PLO to Beirut.

Those supporting the Begin government's conduct of the war included 20 percent of those polled who described themselves as pro-Labor Party.

Begin and Peres had an acrimonious exchange over this issue Tuesday before the Knesset Foreign Affairs and Defense Committee. The next day in a television interview, Peres said a national commission should be formed to investigate "how a campaign that was to have been limited had expanded into a long war."

Begin replied that as soon as he received such a formal demand from the Labor Party "a commission of inquiry on the behavior of some of the opposition during wartime will be established by the government."

In the last Israeli national elections, Begin's Likud Bloc won 48 Knesset seats and the Labor Party 47 seats. Like all Israeli prime ministers before him, Begin was forced to form a coalition with several smaller parties in order to come to power.

Last spring, the strength of the coalition appeared to be eroding and Begin barely survived two confidence votes in the Knesset. But since the war, the government coalition has absorbed a small ultranationalist party and now enjoys a solid majority of 63 Knesset seats.