The Reagan administration strongly pushed Lebanon's Christian leaders to grant Syria a major role in Lebanon's domestic politics as a way of ending the fighting in Lebanon and of opening a new dialogue between Damascus and Washington on improving relations, a senior U.S. official said today.

Interviews with Lebanese and American officials establish that there has been a radical shift in U.S. Middle East policy, which had previously sought to isolate Syria. Since President Reagan's new special envoy, Robert C. McFarlane, took charge two months ago, the United States has sought a political accommodation with Syria as the first step in reducing Russian influence in the Middle East, according to the accounts provided here.

The implications of the shift, which reportedly produced deep divisions within the administration in Washington, suggest that the United States has subordinated helping President Amin Gemayel maintain Lebanon's traditional balances between Christians and Moslems to these goals:

* Avoiding the outbreak of a new Arab-Israeli war that could be sparked by clashes between Syria and Israel in Lebanon's Bekaa Valley.

* Keeping Soviet influence over Syria to a minimum.

* Finding a way to phase out the presence of U.S. Marines in Lebanon.

* Although senior U.S. officials here still pledge allegiance to the May 17 Lebanon-Israeli agreement providing for the withdrawal of all foreign troops from this country, all indications here are that they are quite willing to let it die if that becomes necessary.

"It's just as well that it remains frozen for some time," one American official here said of the accord.

U.S. officials acknowledge that McFarlane's approach has been a carrot and stick one, combining a new willingness to accommodate Syria's insistence that it gain influence over the affairs of Lebanon with salvos from U.S. warships against positions held by Syria's leftist allies.

Among those most confused about U.S. motives here are leaders of the Christian establishment who, gleefully and quite mistakenly, believed the naval gunfire into the mountains here last week meant the 6th Fleet had come to ensure their dominance.

They are now in dark depression as they have come to realize that the terms of the latest agreement for a halt in the fighting here--Lebanon's 179th cease-fire agreement in eight years of warfare--permits, with the assent of American and Saudi emissaries, some kind of a role for Syria in the internal affairs of Lebanon.

A cease-fire negotiated with the participation of Saudi Arabia, Syria and the United States went into effect at midnight Sunday ending three weeks of fighting between Syrian-backed Druze forces and the Lebanese Army. Members of the multinational peace-keeping force, including U.S. Marines, also had come under fire.

In the face of bitter, stunned resistance, the McFarlane mission has bluntly informed the frail government of Lebanese President Gemayel and his Christian supporters that their choices are limited and that the American-led efforts during the past month here represent their last chance for regaining their country.

Lebanese have been told that they will either accept some Syrian influence over their affairs and an enhanced role in the government for Syria's Moslem allies in Lebanon or they will have to accept this country's permanent partition, U.S. officials said.

Christian Phalangist militia commander Fadi Frem, reflecting the views of other Christian leaders, said in an interview: "We are worried about a package deal for which Lebanon would pay the price. We have some signs a package deal is being worked out between Syria and the United States."

Camille Chamoun, 83-year-old Christian patriarch, expressed dismay at the reports, commenting that "they are asking us to be a satellite of Syria which is a satellite of the Soviet Union" and vowing Lebanon would resist the American pressure by all possible means.

"Are we an independent and sovereign country or no?" he asked. "If so, then we have an independent and sovereign policy" and should not "be considered fundamental to the security of Syria."

Pierre Gemayel, 78, father of the Lebanese president and founder of the Phalangist Party, said he was "disappointed" with the United States and warned that a Syrian-American deal would constitute an American "betrayal" of Lebanon.

"You don't abandon a friend like that, one who has put all his confidence in you," he said during an interview at his apartment in east Beirut.

According to accounts here, President Reagan made the decision to seek accommodation with Syria when McFarlane replaced long-time American trouble-shooter Philip C. Habib. The Habib mission had devoted its efforts to securing an agreement with Israel for withdrawal of its forces from Lebanon, accepting the vague Syrian assurances that it would not block the accord and would leave also.

Vehement Syrian objections to the Lebanon-Israel agreement and the refusal of Syrian officials to even see Habib after it was negotiated prompted a reevaluation of American aims here and a change in policy, according to the accounts.

It is said here that Reagan decided that Syria had legitimate historic and security interests in Lebanon that should be accommodated.

In Washington, there were serious qualms about dealing with Syria, dividing the reluctant State Department from the White House. Here there have been indications of the same division, although since McFarlane arrived in late July it has been made clear that he speaks for the president.

One U.S. diplomat here described the tactics of Syrian President Hafez Assad as "rolling objectives." One of the Lebanese Christian leaders here put it differently.

"Assad is a smart man," he said. "He doesn't tell you what he wants. He makes you guess. Maybe you will offer him more than he expected to get."

That belief has prompted the new U.S. strategy here. The United States has sought to keep the Lebanese Army and Gemayel's government in business, but it has also been careful about how far it goes in that support. When the USS Virginia and the USS John Rodgers fired to prevent the Lebanese Army's loss of Suq al Gharb last week, there was careful calculation about the measure of support.

In response to dire Lebanese Army assessments of the situation, the United States not only had readied its naval guns but also had put aloft A4 and A6 jet fighters from the aircraft carrier USS Eisenhower, according to U.S. officials. It was decided that the planes would be too much of an escalation of force, so they were recalled and jettisoned their load in the Mediterranean.

U.S. officials here believe that Syria is mostly concerned with the proximity of Israeli guns, but they also say they think Assad is eager to capitalize on his current position to gain a leadership role in the Arab world.

There also appears to be a U.S. presumption that the deep Israeli domestic dissatisfaction with the entanglement in Lebanon and Israel's serious economic difficulties have caused Israel to scale back ambitions for Lebanon. According to this view, Israel might be satisfied with a reigning in of the Palestine Liberation Organization. There are strong hints here from U.S. officials that the United States has sent this signal to the Syrians.

Washington Post correspondent David B. Ottaway reported the following from Beirut:

Lebanon's Christian leaders reacted angrily today to reports circulating here that Syria and the United States have secretly reached an agreement allowing the Syrians to resume playing a major role in the country.

In three separate interviews, Frem, the Christian Phalangist militia commander, and the two leaders of the Christian Lebanese Front--Pierre Gemayel and Chamoun--expressed grave concern that Washington was shifting its policy to accommodate Syrian political and military interests here at Christian expense.

The three Maronite leaders, two of whom were among the founding fathers of Lebanon at its independence from France in 1943, were reacting to the comments of a senior American official who briefed foreign correspondents Monday regarding the overall U.S. view of the situation here.

During the briefing, the official remarked that the United States hoped a government of national unity would be formed including Lebanese opposition figures "sensitive" to Syrian political and military interests in Lebanon. In this manner, Syria might be persuaded it could reach its objectives here without having to be "physically in the country" and would withdraw its troops.

"The man who made this statement does not know anything about Syrian intentions," said Chamoun. "We know what Syrian intentions are. We paid for it the last seven years."

The U.S. official's comments printed in the local press have been taken within the Christian community to mean Washington has agreed to allow the Syrians to play a dominant role in the new government and the country as they did before the Israeli invasion of June 1982.

Both Pierre Gemayel and Chamoun said they accepted the need to reexamine the basic power-sharing formula worked out in 1943 giving Christians the upper hand in the government and civil service and assuring them the right to choose the country's president and Army chief among other key posts.

The cease-fire accord signed Sunday provides for a national reconciliation committee of leading Lebanese politicians who are supposed to revise this formula and give Moslems a more equitable share in the government.

Pierre Gemayel, while saying he would accept "a little change perhaps," made it clear he was still dead set against any major reforms in the 1943 pact.

"We are ready to abdicate a little power but on the condition that this does not change the essential," he said. "It would be very dangerous to touch that, to reopen the question. The Christians will ask for guarantees and the Moslems would not accept it."

Chamoun indicated he was ready for more radical changes but insisted the debate should take place within the government and parliament and not inside an independent congress of Lebanese political chiefs where Syria and Saudi Arabia would be represented, as called for in the cease-fire accord.

Despite statements by Chamoun posing conditions for his participation in the congress, both he and Pierre Gemayel said today they were ready to attend but had not yet been formally invited. Both were skeptical that much would come of the effort.