Syria and Palestinian guerrilla movement stepped up their public opposition today to President Sadat's peace diplomacy with Israel, bringing the split in Arab ranks closer to a cold war.

A joint statement by Syria and the Palestinian Liberation Organization published early today branded Sadat as a "traitor" and called on other Arab governments to join in opposition to his government. In a threat apparently directed to Saudi Arabia, the statement warned that Arab countries would be "judged" in the light of their stance toward Egypt.

So far, Syrian President Hafez Assad and PLO Chairman Yasser Arafat have not committed themselves personally to a fight to the finish with Sadat - although their propaganda machines are in high gear against Egypt.

The virulence of the anti-Sadat language and the Egyptian step of closing down Palestinian offices in Cairo went far beyond the tensions in 1975 after Sadat signed the second Sinai agreement with Israel. Palestinian and Syrian criticism at the time prompted the Egyptian leader to gag the Palestinian radio operating in Egypt, but not other steps.

While some Arabs were suspending judgment to hear Sadat's own account of his mission and see what concessions he elicited from Israel for his Arab allies, the deterioration of mutual confidence seemed far advanced.

It is widely assumed here that American diplomacy is actively trying to head off a sharp rift by bringing Syria in line and neutralizing the Palestinians. The formula is, theoretically, simple: at American bebest, Saudi Arabia wields its financial leverage on Sadat's behalf, with Syria and the PLO while Israel advances concessions to entice Syria and the PLO to join the peace negotiations.

Syria feels distinctly vulnerable in a power struggle with Egypt. Over-extended in Lebanon and exposed in southern Lebanon where the Palestinians are at Israel's mercy, Damascus is distinctly worse off than in 1975, when it set fire to Lebanon in reaction to Egypt's Sinai pact.

Palestinian guerrillas are even more uneasy about the prospect of a complete break with Egypt and complete dependence on Syria. The prospect holds unpalatable similarities to the situation that exposed them to a Syrian crackdown a year ago. Palestinians no longer look on Syria as a "strategic ally," but they feel increasingly cornered by unfolding Egyptian policy.

Seizing the opportunity, however, extremist Arab forces, who already opposed Sadat and certainly rejected any idea of negotiations with Israel, have swung into action, hoping to bring Syria into their camp and polarize the Arab world by forming a broad radical Arab alliance that would isolate Sadat and contest American influence in the Arab world.

A top PLO official, Abu Luff, who handles external relations and has known Soviet sympathies, flew to Moscow today in search of Soviet support following a Syrian-Palestinian strategy session yesterday.

The Palestinians news agency reported that Algeria was trying to organize a "counter summit" of radical Arab governments - Syria, Iraq, Libya, South Yemen and the PLO.

The key stumbling block holding up a credible Arab radical front is the vicious enmity between Syria and Iraq. Unless they close ranks, Syria is vulnerable to Iraq on one side and Israel on the other. As a condition for its support Iraq has always insisted on Syria's renouncing a negotiated settlement with Israel and permitting Soviet-equipped Iraqi troops to take up stations inside Syria.

Another restraining consideration is Syrian recognition that radical Arab opposition to Sadat would give the Egyptian leader a pretext to proceed to a separate peace with Israel. No Arab doubts any longer that Sadat is a man who could do it.

Whatever Sadat says, the impact of his trip to Jerusalem - recognizing Israel, recognizing Jerusalem as the capital, defusing future boycotts, promising no more wars - outweighted the impact of his irreproachable speech, commentators point out. In their view, Israeli Foreign Minister Moshe Dayan was obliged to deny these very points today in an effort to shore up Sadat temporarily.

Many Syrian and Palestinian analysts objected to Sadat's moves on pragmatic grounds. "He played an Arab trump, recognition, which he should have saved to take a trick," an editor said.

It is not lost on Syrian and Palestinian thinking, however, that Sadat enjoys a stronger short-run position partly because he has an inherently more stable power base than Syria or the PLO.

Agreeing to a more radical line will represent yet another policy shift for Assad, whose own regime has suffered the inevitable erosion of popularity that goes with holding power in Damascus.

For Arafat, the rise in influence of more pro-Soviet Palestinians - a familiar PLO short-run tactic - is an uncomfortable long-term prospect. Arafat, in particular, already has come under mounting personal attack for his own resemblance to Sadat in practicing secretive, personal diplomacy.