The growing new relationship between two of the oldest Arab antagonists in the area -- King Hussein of Jordan and President Hafez Assad of Syria -- has made a negotiated Middle East peace along the lines envisioned by President Reagan highly unlikely, according to senior Syrian officials and western diplomats here.

Although this view differs from the guarded optimism being expressed in Washington and Amman, there is a consensus in this capital of Arab "rejectionism" that the reconciliation between Hussein and Assad spells doom, not hope, for the immediate peace process.

Just how accurate that analysis is will become clearer when Hussein visits here -- probably in the coming weeks -- to seal the reconciliation that began in October with a meeting between the two nations' prime ministers in Riyadh, Saudi Arabia, and continued with their exchange of visits in their own capitals.

Officials in Washington and Amman have sought to put the best face possible on the surprise development of traditionally moderate Jordan seeking to make peace and common policy with radical Syria, the leader of the Arab world's "rejectionist front of nations opposed to compromise with the state of Israel.

These officials have sought to portray the rapprochement as a move that actually widens the potential participation in any future Middle East peace talks. They note the maxim that "in the Middle East you can't make war without Israel and you can't make peace without Syria."

This explanation, however, does not play well here in Damascus among Syrian officials and the foreign diplomats and analysts who deal with them. Conversations with Syrian officials, western and Arab diplomats, and international peace mediators during a recent visit here reveal a rare consensus that the Jordanian-Syrian rapprochement does not mean that Syria has been brought into the U.S.-brokered peace process but that it has begun to draw Jordan out of it.

Rather than the Syrians softening their hard line against any bilateral, or piecemeal, negotiations between one Arab state and Israel along the lines of the Camp David talks, officials and diplomats here agree, Jordan has been forced to accept the Syrian position that the only way to negotiate with Israel is as an Arab bloc, operating from a united stand backed by strong military force.

Proof of just how much Hussein has caved in to the Syrians, Syrian officials here say, is to be seen in the uncharacteristic apology to Syria that he made last month on the eve of his foreign minister's departure for Damascus.

Just before his new, generally pro-Syrian prime minister, Zaid Rifai, flew to Damascus to meet with Syrian Prime Minister Abdul Rauf Qasim, Hussein sent Rifai an open letter that was published in Jordan's state-controlled press. In it, Hussein said he had been wrong in his previous denials that Jordan for years had harbored Moslem fundamentalists opposed to Assad.

In Assad's 15 years in power, the major internal threats to his Baath Party rule have come from the fundamentalist Moslem Brothers, who have contested both his secular socialism and his minority Alawite Moslem sect's domination over the majority Sunni Moslems.

One of the major recent points of contention between Assad and Hussein has been Assad's claim that Moslem Brothers, responsible for assassinations of Syrian officials and for several abortive anti-Assad uprisings, had been given haven in Jordan and allowed to operate from there. Hussein had repeatedly denied the charge.

But in his letter to Rifai, Hussein said that he had been deceived. "It came to light that some of those who were connected to the bloody incidents in Syria were present in Jordan and were taking refuge in houses of a deviant minority cloaking themselves in the gowns of Moslem religion," he said.

"The king played the role Syrians expected him to play -- that of a small king of a small country," one western ambassador here said. He added that Syrians with whom he had talked were puzzled that Hussein had gone further than even Assad might have expected him to.

"He admitted the facts and proved we were right all the time," said Kamal Assad Elias, one of Assad's longtime aides. "But what matters to us is that he should agree not to make a separate deal with the Israelis and agree to adhere to the resolutions of the Arab summit" at Fez, Morocco, in 1982.

Such an agreement, contained in the joint communique issued after Rifai and Qasim met here, means, Syrian officials said, that the only way for peace negotiations to take place now is through the sort of international conference advocated by the Fez summit.

That would provide for representation of all Arab governments plus the PLO, with representatives of the major powers, including the Soviet Union. That is a far cry from the Reagan administration's hopes for direct negotiations between Israel and Hussein, with or without Palestinian representatives.

The new relationship between Amman and Damascus also has pulled the rug out from under Yasser Arafat, who has banked heavily on his own painfully forged alliance with Jordan in the past year to counter the hostility of Assad. The Syrian president personally loathes the PLO leader, who has consistently refused to let his organization come under Damascus' control.

Although officials of Arafat's mainline PLO have not criticized the new relationship between Syria and Jordan, their concern is clear from their frenetic diplomatic activity since Rifai visited Damascus.

First Arafat made an unplanned visit to Amman to meet with Hussein. Last month he convened a special meeting of the PLO Executive Council in Baghdad. Officially, the Jordanian-Syrian rapprochement was not on the agenda, but no one here thought Arafat could convene his closest aides and ignore discussing possibly the most significant new factor in the Middle East since the 1982 Israeli invasion of Lebanon.

What motivated Hussein to shift his ground so dramatically has yet to be clearly explained. The supposition among analysts here is that he may have decided that he was much too isolated in the Arab world over his position on negotiations, with too little support from the United States or from other moderate Arab regimes, such as Saudi Arabia.

Elias said, "Perhaps King Hussein recently noticed the line he was following was not getting anything in return. He was going down a really dangerous path -- the more concessions he was giving, the more concessions he was being asked to give -- with nothing in return."

"After his return from the United States [in October] he was denied the weapons he had previously sought," Elias said. "He got no sign that the United States could help him get a comprehensive and just peace in the Middle East, so he had to rethink everything. We are glad he did and welcome him back to the mainstream of Arab solidarity."