Col. Muammar Qaddafi's surprise call for a total constitutional merger of Libya with Syria drew quick approval today from Damascus.

While organized crowds marched through the streets of Tripoli during the course of the day shouting their support of Qaddafi's latest pan-Arab proposal, Radio Damascus announced that Syria "welcomed" the Libyan leader's idea and that President Hafez Assad had telegrammed Qaddafi to tell him of his agreement.

"This unity appeal has struck an immediate favorable response among us," Assad was reported as saying in his telegram. "We stand together on the path of unity . . . We stretch out our hand to shake yours in order to begin at once the work to bring about this great goal."

Senior diplomatic observers here said there were good reasons for taking the idea as more than just another exercise in rhetoric despite its implausibility, given the failure of all such attempts to unify the Arab world.

While details of the agreement between Tripoli and Damascus have yet to be negotiated, these diplomats say that Qaddafi's speech last night on the 11th anniversary of his revolution had been preceded by secret general discussions on the union by emissaries from the two capitals.

The fact thaft Qaddafi's speech was carried on live television in Syria as well as Libya, and that the only chief of government who deigned to attend Qaddafi's celebrations here was Syrian Prime Minister Abdel Rauf Kassem, was the tip-off that the deal had been struck. Assad's telegram of assent was a final confirmation.

Just how far toward unity the countries actually go remains to be seen Arab sources here, including some Libyan officials, express doubt that any real union is possible between regimes as different as those of Assad and Qaddafi and in two countries as far removed from each other as Libya and Syria.

Whatever the plan that emerges from negotiations -- merger, union or loose cooperation -- it provides an answer to a mutual necessity in both countries to unblock stagnant foreign and internal political situations, according to diplomatic observers here.

"Both Syria and Libya are isolated from their fellow Arabs, and both are undergoing internal tensions," noted one Western ambassador here. "By getting together, or seeming to get together, at least, both hope to break out of their respective political straitjackets."

Assad's Syria certainly is in need of whatever friends it can get, especially rich ones like Libya. The government has been racked by internal dissent and its economy is in tatters. Its peacekeeping operations in explosive Lebanon have tied down enough of its army to make Damascus worry about being able to maintain its war readiness against Israel to the south.

Egyptian President Anwar Sadat's peace with Israel has successfully removed Egypt, its Army as well as its strategic southern front, as a threat to Israel, leaving Damascus vulnerably exposed in any new Arab-Israeli war. t

It was this factor that last year brought Assad to try to smooth differences with his oil-rich neighbor and archrival, Saddam Hussein of Iraq. Efforts to reunify the Syrian and Iraqi wings of the Baath Party ruling the countries collapsed in acrimony and mutual recrimination only a month ago. Both Syria and Iraq expelled each other's ambassadors and diplomats from their respective capitals.

It was a disaster that left Syria more isolated than ever before, making some sort of cooperation with Libya sudddenly attractive, even given Assad's past reservations about the unpredictable and highly emotional Qaddafi.

For Qaddafi, Assad's wooing could not have come at a better time. His own messianic foreign policy has been a shambles. Adventures in Uganda, Chad and neighboring Tunisia have proven disastrous, both internationally and internally.

His bitter feud with President Sadat has both deprived Libya of much needed Egyptian technicians as well as tied down a large part of his Army on the border with Egypt where Sadat keeps an Army of 50,000 well-trained troops.

Even his efforts to cultivate the tiny island of Malta as a base of influence in the Mediterranean have collapsed around his ears in a bitter dispute over oil exploration rights on the continental shelf separating their two countries.

Like Assad, Qaddafi, too, has been faced with internal tensions created by his own "cultural revolution" against the entrenched elite, in government as well as society at large, who have run the nation since independence. There has lately been a rash of small demonstrations against his rule, and though the government sloutly denies it, there have also been reports of an Army mutiny in Tobruk.

Qaddafi is still caught up in his own personal vision of himself as the heir to the late Egyptian president Gamal Abdel Nasser, who has a dream of uniting the Arab world from -- as he said last night in announcing his merger proposal -- "Morocco to Bahrain." Thus, he naturally welcomed Assad's approaches on cooperation as yet another chance to try and darn his mentor's pan-Arab mantle.

Qaddafi's own past failures at forging unity -- with Egypt in 1972 and with Tunisia in 1974 -- are cited by other Arab diplomats as reason for their skepticism about the current plan. They also note Syria's own failures in merger efforts with Iraq and Egypt earlier in the 1960s.

Still, Assad is hoping that Qaddafi will, as promised, kick in some on his nation's $16 billion a year in oil revenues to boost Syria's stricken economy in return for a chance to present himself to a skeptical Arab world as the new savior of pan-Arabism.