Secretary of Defense Harold Brown today sought to reassure Saudi Arabia that the United States remains a reliable ally despite the failure of American aid to save the shah of Iran.

Beginning a 10-day Middle East trip,,brown said Washington is willing ans able to stand by its friends "in peace or war."

In separate meetings with Crown Prince Fahd and Prince Sultan, the defense minister, Brown pledged that the United States would thke appropriate action -- including the use of armed force if necessary -- to help Saudi Arabia resist an external military threat.

The meetings with the princes, both brothers of King Khalid, were private, but the substance of Brown's remarks was relayed to reporters by a senior U.S. defense official.

In a speech to cadets of the Saudi millitary and air academies Brown said, "We share your commitment to peace rather than war. But no one should doubt our willingness and ability to be good friends in peace or war."

Standing next to a concrete parade ground in sunny, 78-degree weather, Brown predicted a future of increased military cooperation between Washington and Riyadh.

"We can provide the best training and equipment in the world," he said. "We will do so.

"We can provide the extra strength needed to meet a foe from outside the region," he added. "We will do so."

The senior defense official who described Brown's mission to the reporters said the secretary was careful to pledge U.S. aid only for resisting outside aggression. He said there is little the United States could do to prevent the Saudi royal family or any other government from going the way of the shah if the threat came from within as it did in Iran.

"With respect to internal security, there is limited help that an outside country can give," the official said. Brown arrived this morning after a 14-hour flight from Washington. He plans to visit Dhahran in Saudi Arabia's rich oil fields Sunday and will later go to Jordan, Israel and Egypt.

The U.S. defense chief urged the Saudis to expand their military aid to moderate states in the region such as Egypt, North Yemen and Oman.

The senior defense official, however, said it would be impossible for Saudi Arabia, despite its vast oil wealth, to assume a role of self-appointed policeman of the Persian Gulf region such as the one Shah Mohammad Reza Pahlavi had staked out for Iran.

"I don't think the Saudis themselves would claim that their population or state of military development is such that they would be equivalent to Iran," the official said.

At each stop in his trip, Brown plans to discuss the continued supply of arms. All four governments are already customers for U.S. armaments, with Saudi Arabia the biggest purchaser last year.

But Brown's task of reassuring the four countries on the value of U.S. support may not be easy. The shah fell despite more than $8 billion worth of U.S. arms, including some of the most sophisticated weaponry in the American arsenal. Brown must convince the four nations that while military might alone will not ensure a government's survival, weapons can sometimes help.

"If the Saudis say that F14s or F15s don't solve a country's internal religious or political problems, they will only have beat the secretary to saying the same thing," the official said.

Nevertheless, the official said, Btown warned that the Middle East faces the possibility of aggression from outside the region, probably supported by the Soviet Union, a threat that weapons and military training can help meet.

"You have to solve all of the problems that can bring you down, not just some of them," the official said.

Brown also sought to convince the Saudis that even should their faith in the value of U.S. aid waver, they would find the Soviet Union to be a far less sympathetic military and political patron.

He reminded the princes that in Afghanistan "a government already friendly to the Soviets was overturned by an even more avidly pro-Moscow regime."

"Just following a pro-Soviet line is no protection against a Soviet-backed coup," he said. conditions.

The Sunday before election, pro-life groups passed out the leaflets in Catholic church parking lots across the state. When Clark was defeated, 52 percent to 48 percent, Brown was quick to claim credit.

"We wanted to prove that the pro-life vote was for real last fall," he says. "We did that. We showed that people will cross party lines to vote on this single issue."

Brown claims local grous in more than 20 states are considering starting political action committees modeled after LAPAC. "We're going to haunt to death anyone who runs in 1980," he says. "We can and will make it so difficult that they'll have to give us our amendment."

No one doubts the fervor of those in the pro-life movement -- or their ability to mobilize quickly. (Brown claims it can generate 50,000 letters to Congress almost at the drop of a hat.) And a Right to Life Party candidate for governor in New York State last fall attracted more than 100,000 voters, enough to automatically qualify the newly formed party on future ballots.

But many observers still question their real power at the ballot box.

"I thin? they have some influence, but I don't think too many people will vote against me because of my position on one issue, particularly if they have heard what I have to say about it," says Rep. Fisher, who has been passed out in chouches before his last two elections.

Even Iowa -- supposedly the most dramatic example of single-issue politics in last fall's election -- is not as clear-cut a case as some believe.

When the Des Moines Register sur veyed voters leaving the polls on election day, it found 8 percent of them said they'd voted for Jepsen because of his stand on abortion. In an election decided by only 28,000 votes, this would be enough to tip the balance. But in analyzing its own poll results, the newspaper concluded "the abortion issue was not decisive."

There's no denying, however, that the pro-life -- and other single-issue -- groups have a goodly number of politicians scared half to death. "These groups are a dangerous trend in American polities," complains Bayh, echoing the sentiments of many liberals.

The great irony about this is the groups are gaining political clout at a time when every major poll taken on the subject shows that a majority of Americans support a woman's right to choose whether or not she wants an abortion.

"We are not outnumbered," complains NARAL's Mulhauser. "We are being out-organized."