Four years after Israel invaded Lebanon in an effort to destroy Chairman Yasser Arafat's Palestine Liberation Organization, his weakened forces are again fighting for their physical and political lives -- this time against fellow Arabs.

In Beirut, fighting pits Arafat loyalists and even nominally anti-Arafat Palestinians against Shiite Moslem Amal militimen in a rerun of last year's so-called "war of the camps" where many Palestinian refugees still live.

Rightly or wrongly, many Arabs automatically assume that Syria is backing the Amal drive in a renewed effort to prevent Arafat from reestablishing anything approaching his pre-1982 state-within-a-state status in Lebanon.

In Jordan, with apparent official complaisance, Atallah Atallah, a cashiered PLO military intelligence chief, claims to have ousted Arafat as PLO chief. Some observers conclude that Jordan's King Hussein at least tacitly encouraged Atallah's actions to signal continuing displeasure with Arafat for allegedly causing the breakdown in February of their year-long joint effort to reach a Middle East peace agreement.

The timing suggests that the monarch is seeking to burnish his image with Congress before his arrival in Washington, where he hopes to persuade U.S. officials to reconsider arms sales to his country now that he has broken with Arafat, according to analysts and diplomats.

Hussein left Paris Wednesday for Boston to attend his twin daughters' graduations, United Press International reported. He is scheduled to meet with President Reagan in Washington on Monday.

Arab analysts have suggested that Jordan and Syria may be acting in concert to bring Arafat to heel. Their aim is described as part of a larger plan to foster reconciliation between Syria and Iraq, whose capitals are the seats of violently opposed branches of the Baath (Arab Renaissance) Party.

Such joint action would be calculated to please the United States and Britain, whose prime minister, Margaret Thatcher, recently endorsed Jordanian and Israeli arguments for ditching the PLO for more pliant Palestinian leadership in any future peace negotiations.

But so far, Iraqi President Saddam Hussein has shown no disposition to stop providing Arafat with facilities in Baghdad -- a way for Saddam Hussein to demonstrate the PLO chairman's ability to operate despite Arafat's strained relations with Syria.

Many of Arafat's Palestinian critics are convinced that even if his longtime financial and political protector, Saudi Arabia, were to turn on him, such concerted action by Arab states at most would weaken him, rather than remove him from leadership. These critics reason that Arafat and Syria are using the Beirut fighting as part of a deadly poker game to test each other.

As one Palestinian official in Damascus said recently in predicting the renewed fighting: "Arafat knows he's exaggerating his claims about being back in Beirut stronger than in 1982. But that is just the kind of nightmare scenario he thinks might press Syria into reconciliation talks" three years after Syrian President Hafez Assad unceremoniously ejected Arafat from Damascus.

So far, Assad has refused to entertain any reconciliation, vetoing suggestions in private while publicly arguing that principles rather than personal differences are at stake.

Privately, Syrian officials acknowledge that Assad, like Hussein, has failed to stamp out the PLO's legitimacy for ordinary Palestinians, especially those in the Israeli-occupied West Bank and the Gaza Strip.

Moreover, Assad's stand has put him at odds with his Soviet allies. Now that the U.S.-backed peace initiative with Jordan has collapsed, the Kremlin has sought to improve its once strained relations with Arafat. Underscoring the Kremlin's change of heart, Soviet leader Mikhail Gorbachev received Arafat in April in East Berlin during the East German communist party congress.

In turn, much to Syrian annoyance, Arafat has publicly supported a Soviet-backed call from Algeria for reconciliation talks among all the badly split Palestinian factions.

With Arafat's mainstream Fatah refusing to sit with pro-Syrian factions that took up arms against his wing of the PLO and with Assad refusing to let his Palestinian proteges attend, plans to convene a meeting in Algiers in early May went by the boards.

But such is the deep-seated longing for even a semblance of unity in Palestinian ranks that critics of Arafat's often autocratic leadership may yet forgive -- if not forget -- what they see as his past excesses. For all the aging Palestinian leaders are aware that their long collective stewardship has weakened the organization whose continued existence often has seemed more important than attaining an independent Palestinian state, their proclaimed objective.