A high-level political assassination and veiled Syrian threats have chipped away at Chairman Yasser Arafat's recently reasserted leadership over the Palestine Liberation Organization.

This continuing erosion of Arafat's authority is in turn undermining hopes that Arafat and Jordan's King Hussein will be able to reach agreement on a joint negotiating plan that would give new life to President Reagan's stalled 1982 Middle East peace plan, which seeks to wed Jordanian and Palestinian interests in negotiations with Israel.

The procedural victory Arafat won in November by convening the Palestine National Council here in defiance of objections by Syrian-backed dissidents was followed in December by the shooting of Fahd Kawasmeh, a key moderate on the PLO Executive Committee who had pressed for the Amman meeting.

The Dec. 29 killing appeared a clear warning against renewed Jordanian-Palestinian efforts to work out a negotiating position, and since the assassination, doubts have been raised that Arafat and Hussein alone can work out a united stand without Syria's approval.

Such pessimism, in turn, reinforces the sense of stalemate in efforts to begin talks with Israel over the future of the West Bank. There is a perception among Arabs that the United States is indifferent to any renewed Middle East peace initiative. Falling oil prices have blunted the Arabs' main diplomatic weapon; meanwhile they assume that Israel's weak coalition government is unable to accept compromises to get talks started.

At what many Palestinians describe as a very dangerous time and the PLO's lowest ebb, the feud between Arafat and Syrian President Hafez Assad and the Palestinian factions he backs fuels doubts about Arafat's effectiveness. The Palestine National Council meeting in Amman is interpreted in retrospect as less a mandate to negotiate with Jordan than a dangerous challenge to Syria.

Palestinians in and out of the PLO, Jordanian officials and diplomats again question Arafat's ability to reach a joint position with Jordan.

The national council, a kind of parliament-in-exile, was "great atmospherics," one western diplomat remarked, "but produced no movement."

Symptomatic of the gloom here is the realization that the now-formalized split between pro- and anti-Arafat forces has increased pressure on Arafat to be seen acting decisively.

"I'm not talking about taking back the West Bank tomorrow," a prominent West Bank intellectual said with a wistful smile, "but Arafat has got to show that he can make something work, not just more words, and within a few weeks, not months."

Yet, neither Jordanian nor Palestinian leaders here expect an outcome of their own exploratory talks on a joint approach to Israel before two or three months.

That leisurely pace is understandable given the complexity of the negotiations and the failure of similar talks two years ago conducted in generally more positive circumstances.

"The king still thinks he can pull it off," a diplomat said, "but I'm skeptical unless Arafat is willing to settle for an awful lot less than he's ever said he will."

"The king keeps insisting on a joint Palestinian-Jordanian delegation and keeping the defense and foreign affairs portfolio in a confederated state," he said. "That seems to leave not much beyond an honorary title for Arafat."

Both sides stress the need to make haste slowly to such an extent that cynics speculate they hope some unpredictable event will alleviate their plight or at least distract attention from it.

Into this process are injected the fears caused by Kawasmeh's assassination.

Kawasmeh is believed to be the first Executive Committee member killed by fellow Arabs.

The former mayor of Hebron, on the West Bank, he was deported by Israel in 1980 for pro-Palestinian activities. He was recently elected to the Executive Committee, the inner sanctum of the PLO, and was instrumental in persuading Arafat to convene the national council in Amman without further delay to commit the PLO to negotiations in full view of the West Bank television audience.

As far as is known, Kawasmeh's killers have not been definitely identified or arrested. But Khalil Wazir, Arafat's top aide, also known as Abu Jihad, charged in an interview that the assassination had been directed from Damascus by Brig. Mohammed Khouli, head of Syrian Air Force intelligence.

Wazir saw no possible improvement of relations with Syria "in the immediate period." He said Syria had "declared war on PLO legality," his term for the Palestine National Council and Executive Committee. He said he expected that Syria "will continue its attacks and assassinations in the coming months."

But he pledged that "on our side we will not be drawn into a lake of blood" by ordering reprisal assassinations against the Syrian-backed Palestinian dissidents he accused in Kawasmeh's murder.

Wazir dismissed as "something we expected" Assad's implied threat last week to withdraw Syrian recognition of the renewed PLO.

He also predicted renewed Syrian pressure on the Damascus-based Democratic Alliance -- principally the Palestine Communist Party, the Democratic Front for the Liberation of Palestine and the Popular Front for the Liberation of Palestine. These groups have sought to bridge the gap between Arafat loyalists and the Syrian-backed dissidents.

Mindful of such pressures, Ahmad Jamal of the Democratic Front said the assassination of Kawasmeh made it "more difficult but also more urgent and important" to reconcile the feuding Palestinian parties.

But Mohammed Milhem, a former West Bank mayor deported with Kawasmeh and also on the Executive Committee, said: "Reconciliation comes only when both parties feel no one is going to win."

If interviews with Milhem and Jamal are any yardstick, leading Palestinians are very much personally concerned about Kawasmeh's assassination.

Milhem stared from his ground-floor window, obviously upset by a noisy argument between the recently assigned Jordanian guard and a passer-by. And Jamal's neighbors rang his doorbell, nervously inquiring why an unidentified car -- rented by visiting journalists -- was parked near his door after dark.

But beyond such personal considerations, many other Palestinians are wondering how the present rift with Syria can be ended while Arafat remains as PLO chairman.

"He's got tremendous personality, character and rapport with the Palestinian people," said a former close associate, "but rapport is not enough. Only muscle will persuade Israel to deal with the PLO."

One prominent West Bank Palestinian said that after years defending the sacrosanct principle of Palestinian decision making on issues affecting the Palestinian people, "we might just have to lose lots of independence. But we must find a way to deal with Syria."

Without Syria, the PLO and Jordan did not have the strength to impress Israel, in Assad's analysis, and the Palestinian reluctantly conceded that he agreed with it.

"What can we do?" he asked. "We're stuck."

In this environment of frustration and pessimism, Arafat's doubters are no longer limited to the Syrians and their Palestinian dissident allies. One West Bank intellectual now living here suggested that 20 percent of the Palestinians now question Arafat's leadership, whereas a decade ago, he said, not one-tenth of 1 percent would have expressed any doubts -- and even a year ago such conjecture would have brought howls of protest.

Yet almost all Palestinians appear still to be convinced that Arafat will remain at the helm. In the past, that was said jubilantly, even defiantly, especially when Israel or Arab governments sought to dislodge him. Now, increasingly, it is said with just a touch of resignation.

Like many another minority in the Middle East, the Palestinians appear unwilling to change leaders, no matter how deep-seated their misgivings, for fear their enemies would interpret any such move as fatal weakness.

Even Palestinians long critical of Arafat's freewheeling ways, such as leftist followers of George Habash's Popular Front for the Liberation of Palestine and Naef Hawatmeh's Democratic Front for the Liberation of Palestine, stoutly oppose Syrian-backed dissidents' efforts to remove Arafat, fearing that after such a move, Syria might feel free to remove Habash or Hawatmeh.