By lopsided numbers Americans believe President Reagan's hard-line approach to foreign affairs is creating new respect for the United States overseas, according to a new Washington Post-ABC News public opinion poll.

Three in every four Americans feel that way, according to the poll, even though a majority also feels that Reagan's policies are creating anti-American feelings in many countries.

Overall, the public gives the president high marks for his conduct of foreign affairs and continues to support him strongly on his plans for a military buildup. At the same time, a hard core of about one-third of the population fears Reagan's stance toward the Soviet Union is increasing the chances of war, and many citizens oppose specific positions Reagan has taken on a number of key issues.

Reagan has been sharply criticized from several quarters recently for having an erratic and unclear foreign policy; some have called it his weakest suit as president. By 52 to 31 percent, however, those interviewed in the Post-ABC News poll agree with the statement that Reagan "has a clear foreign policy which most countries can understand."

Among the poll's chief findings:

A majority of Americans are opposed to the sale of Airborne Warning and Control System radar planes to Saudi Arabia, largely because of a widespread conviction that Saudi Arabia is not a reliable ally. Many citizens on both sides of the issue, however, say they do not feel strongly about the sale and that theiropinions are subject to change.

The Middle East in general, and not the Soviet Union, is the leading source of concern to the American people. For every person who cites the Soviet Union as this nation's biggest foreign affairs problem, two others cite the Middle East.

Americans overall have come to view Egypt almost exactly as they do Israel. Almost half the public thinks of both nations as trustworthy allies of the United States, 10 percent see Israel but not Egypt as trustworthy, 8 percent see Egypt but not Israel that way, and 13 percent see neither nation as a reliable ally.

Forty percent agree with the Reagan position that the United States should be militarily stronger than the Soviet Union, 46 percent say this nation should strive simply for equality, and another 10 percent say the United States "should limit its military spending" even at the risk of falling behind the Soviets.

Those interviewed by The Post and ABC News tended to be militant on some issues but not militant at all on others. For example, 63 percent say they agree with the statement that "the United States should take all steps, including the use of force, to prevent the spread of communism." But for the first time since Soviet troops moved into Afghanistan in late 1979, a majority of Americans say they are against a return "to the military draft."

Citizens tend to see Reagan as staking out positions that are more aggressive than the ones they would choose, but often they praise the president for such a posture.

A 20-year-old auto garage transmission worker from suburban Chicago, for example, said he was against using force to prevent the spread of communism but felt that Reagan was for it. He gave Reagan a high grade for his handling of foreign policy and said, "I think we are showing everyone that the United States is not going to be pushed around, even if it means war."

On many foreign policy issues, the poll shows, a majority of citizens suffer from a lack of knowledge. Asked which country, the United States or the Soviet Union, was a member of the NATO alliance, 51 percent say they do not know, 47 percent give the right answer, the United States, and 2 percent say the Soviet Union.

In a similar vein, only 37 percent are able to answer correctly that the United States and the U.S.S.R. are the two nations that have conducted "what are known as SALT talks."

In all, only 30 percent of those interviewed answered both questions correctly. Frequently, those who are able to answer both questions hold views that contrast with those who answer only one or none correctly.

Many questions in the poll, however, deal with underlying attitudes--feelings on issues which require no particular knowledge.

One series of questions in the poll, for example, aimed at gauging circumstances under which people would approve of the use of military force. It found strong support for U.S. military action in the event that "some nation takes American hostages, the way Iran did," and similar strong support for taking action if the Soviet Union "tries to put missiles in Cuba."

In addition, a majority say they support U.S. action if the Soviet Union moves troops into Iran or the Persian Gulf. Majorities oppose the use of force, however, to counter any revolution in Saudi Arabia, a possibility raised by Reagan at a recent news conference when he said, "I have to say that Saudi Arabia we will not permit to be an Iran."

Of the instances listed, Americans were most opposed to military action in the event that "war breaks out between Israel and some Arab nations." Twenty-eight percent say they would favor U.S. military intervention in that circumstance and 61 percent say they oppose it.

The new poll suggests that support for Israel, while still strong among Americans, has been waning. The poll asked citizens whether they agree or disagree with this statement: "The United States should cooperate with the Arab nations to insure an adequate oil supply even if that means lessening our ties to Israel."

Thirty-nine percent agree, 43 percent disagree. But as recently as March, in an earlier Post-ABC News poll, only 26 percent said they agreed with that statement and 61 percent said they disagreed.

At the same time, Israel, along with Egypt, finds far more favor with Americans than does SaudiArabia, the one other Mideast nation being courted by President Reagan.

Israel is seen as a reliable ally of the United States by 64 percent of those interviewed and Egypt is seen as a reliable ally by 59 percent. Only 33 percent say that Saudi Arabia is a reliable ally, however, a level equal to the perception of China as an ally.

This sense that Saudi Arabia cannot be trusted is a key factor in the public sentiment against the sale of the AWACS radar planes. Those who see the Saudis as trustworthy support the sale by 61 percent to 32 percent, but those who do not oppose the sale by 68 to 25 percent.

Washington Post polling assistant Kenneth E. John contributed to this article.