The massacre of hundreds of Palestinians in West Beirut has prompted a sharp increase in negative views toward Israel among Americans, according to a new Washington Post-ABC News public opinion poll.

A majority of Americans believes Israel shares significantly in the blame for the massacre by Lebanese Christian militiamen. A great many no longer think of Israel as an ally that would cooperate with the United States in almost any circumstances. And they are focusing their discontent with recent Israeli actions on Prime Minister Menachem Begin.

Overall, Americans remain much more sympathetic toward Israel than the Arab nations, according to the new survey. But this sympathy is not nearly as strong as it was only a month ago, as measured by a Post-ABC News poll in August.

At the same time, the American public, by a ratio of more than 2 to 1, feels that Israel's motivation in recent months has been self-protection, not aggrandizement. Despite the massacre, there remains a general optimism that the removal of the Palestine Liberation Organization from Lebanon will change things for the better in the Middle East.

The poll also found Ronald Reagan dropping to a low point in popularity, with 50 percent expressing disapproval of his handling of the presidency for the first time in a Washington Post-ABC News poll. Sentiment toward Reagan seems tied to a sharp increase in disapproval of his handling of foreign policy.

Concerning responsibility for the massacre, 28 percent of those polled say "a great deal of the blame for the killings should go to Israel" and 39 percent say Israel deserves "a fair amount of the blame."

Only 20 percent say Israel should get only very little or none of the blame.

As often occurs, Americans are sending out mixed and sometimes conflicting signals about their views.

But in thrust, those views are much more skeptical toward Israel than they have been in previous Post-ABC News polls.

Starting in October, 1981, the Post-ABC News poll has asked random samples of the population whether they "regard Israel as a reliable ally of the United States -- one that can be trusted to cooperate with the United States in almost any circumstances -- or not."

The question was repeated in March, 1982, again in August, and once more in the new poll conducted Sept. 24 to Sept. 26.

Last October, 64 percent said Israel was a reliable ally; 24 percent said it was not, and 12 percent expressed no opinion.

In the new poll, 40 percent said Israel was reliable and 45 percent said it was not.

By contrast, Egypt today is regarded as a more reliable ally than is Israel, a sharp change in perceptions over the last 11 months.

The focal point for the changing American sentiment clearly seems to be Begin.

In March of this year, 39 percent of those interviewed in a Post-ABC News poll said they had a favorable opinion of Begin; 22 percent had unfavorable views.

Consistent with the finding that many Americans do not pay much attention to events in the Middle East, 39 percent at that time said they had no opinion one way or the other about Begin.

By mid-August, two and a half months after the Israeli invasion of Lebanon, views toward Begin had become evenly divided: 33 percent approved of him and 32 percent disapproved, with the largest single group the 35 percent who had no opinion.

In the new poll, only 26 percent say they view Begin favorably, and almost twice as many--47 percent --view him unfavorably. Only 27 percent now had no opinion.

As in August, the new poll shows that the minority of citizens who tend to follow events in the Middle East at least somewhat closely are more sympathetic to Israel than are the majority, for whom most overseas events are remote.

But those who pay attention also are more troubled about Israel today than they were a month ago.

Whether this rise in negative views toward Israel and its government will continue cannot be predicted by the Post-ABC News poll.

Mideast crises have caused ups and downs in public opinion in the past.

For the time being, the increasingly skeptical views of Israel appear to be producing troubled questioning of relations between the United States and Israel.

For example, 59 percent of those interviewed agree with the statement that "the United States should stop supplying Israel with military arms." Only 35 percent disagree, and 6 percent express no opinion.

One of the most sensitive concerns among American Jews is the question of whether their own allegiance is more with Israel than with the United States, and the poll shows a sharp split on that:

Q: "Agree or disagree: Most Jewish people in America will support anything the country of Israel does even if it is against the best interests of the United States."

A: Agree, 41 percent; disagree, 53 percent; no opinion, 6 percent.

Americans reject the position taken by PLO chairman Yasser Arafat, who holds the United States largely responsible for the massacre.

In the poll, 2 percent say the United States deserves a great deal of the blame, 10 percent "a fair amount" and 81 percent either "very little" or "no blame at all."

Nevertheless, the poll also shows a sharp drop in support for President Reagan's handling of the Middle East and of foreign affairs in general.

In August, Reagan's handling of Middle East issues met with 48 percent approval and 37 percent disapproval.

Today, disapproval outweighs approval by 45 percent to 42 percent.

Overall, 43 percent say they approve of Reagan's handling of foreign affairs and 45 percent say they disapprove.

That is a reversal from a Post-ABC News poll earlier this month, when 50 percent approved and 35 percent disapproved Reagan's handling of foreign policy.

In addition, Reagan's so-called "popularity rating" was lower in the new poll than in any earlier Post-ABC News poll.

Fifty percent say they disapprove of the way he is handling his job as president, while 46 percent approve.