The victory of Bashir Gemayel's Phalangist Party militia in a round of inter-Christian fighting appears to be pushing his rivals toward a political alliance with Palestinians and Labanese leftist Moslems.

Even by the standard of Lebanon's ever-shifting alliances, the virtual expulsion of the Christian National Liberal Party militia from Phalangist-dominated areas in bloody fighting last week shows signs of leading to a radically new alignment.

Some observers feel this may signal a potential breakthrough for a measure of Christian-Moslem national cooperation that has so long eluded this strife-torn country.

Since the Phalangists' victory, Palestinians and Labanese Moslems have rallied around the Christian National Liberal Party's militia chief, Dany Chamoun.He is the son of the party leader, 80-year-old former president Camille Chamoun. During the 1975-76 Labanese civil war, the Phalangists and the National Liberals fought in a Christian militia alliance against the Palestiniians and Moslems. The war ended in a standoff when neighboring Syria invaded to stop the fighting.

The latest inter-Christian fighting, which reportedly has included cold-blooded killings and torture, has shocked Moslems and Christians alike here. According to the Phalangists, 65 persons were killed, but the National Liberals put the death toll at 5309

Dany Chamoun, 46, told reporters today that he was determined to mount "a political offensive against the Phalangists to save the Christian Lebanon of what he termed "Nazi-style Phalangist occupation."

Phalangists have admitted "some excess in the exercise of [their militiamen's] functions," and said about 10 of their fighters will be court-martialed.

In an interview at East Beirut's Phalangist "war council" headquarters, Bashir Gemayer, 33, justified his military actions as the only way to put an end to chaos and daily confrontations among the heavily armed Christian militias.

His lightining strike against National Liberal storngholds in mainly Christian east Beirut, 15 miles along the northern coast and in the hilly countryside northeast of the capital, stripped his rivals of all military strength.

The elder Chamoun, a hardened political fighter who subdued his political Moslem foes as president in 1958 when he invited U.S. Marines to land in Lebanon, has kept a stiff upper lip on the inter-Christian fighting. He described the defeat of his party's militia as a "massacre." But he has not hinted publicly that he plans to leave the alliance with the Phanlangists, the "Labanese Front."

Leftist sources said the Palestinian leadership in Lebanon has tried to pressure Walid Jumblatt, head of the National Movement, a Moslem leftist group, into accepting Chamoun back in the Chouf district. There the Jumblatts, a Druze clan, and the Christian Maronite Chamoun family had shared political power before the civil war.

Meanwhile, Dany Chamoun has visited Moslem leaders such as Prime Minister Selim Hoss. He also met with former President Suleiman Franjieh.

Franjieh has been declared an enemy of the Gamayel clan since the summer of 1978, when his son Tony was killed along with Tony's wife and baby and 30 other followers by Phalangist gunmen. The former president has vowed revenge against Bashir Gamayel, who he believes was responsible.

Dany Chamoun was also quoted by the pro-Syrian newspaper Ash Sharq today as saying he wished to improve relations with neighboring Syria.

Bashir Gemayel insisted, on the other hand, that Christian areas need to unify militarily.

"The situation had become untenable," he said. "There were almost daily quarrels that developed into major confrontations with heavily weaponry" between Phalangists and the National Liberals.

"It became inconceivable that armored vehicles and tanks remain in the hands of the population like little toys," he added. National Liberal Party militiamen handed over all their heavy arms after they surrendered.

Asked why everyone had to give in to him, Gemayel replied, "Someone had to do it."

Gemayel, who has been accuesed of aspiring to set up one-party rule in a separate Christian state, rejected such charges by saying he is only seeking provisional measures until the state can assume its duites.

However, he added, "We are trying to become self-sufficient to be able to support any kind of war effort. We don't have the choice."

Following the military victory over the National Liberals, a spokesman for Gemayel's militia said the creations of a "national home guard" with a potential force of 40,000 men was envisioned. The main goals of that home guard would be to "liberate Lebanon from all foreign domination," meaning Syrian peacekeeping troops and Palestinian guerrillas.