The highest councils of the Palestine Liberation Organization, after more than a week of public dissension and thinly veiled confrontation, issued a qualified endorsement yesterday of the recently announced plan for joint Jordanian-Palestinian action on Middle East peace that seemed to undermine the initiative almost as much as support it.

At the same time, Israeli Prime Minister Shimon Peres told reporters in Rome that he is willing to go to Jordan to move the peace process forward. He once again invited Jordan's King Hussein to visit Jerusalem. But he rejected the international peace conference that is a cornerstone of the Jordanian-Palestinian plan.

The Soviet Union, which would be a major participant in the conference as contemplated by the Jordanian-PLO plan, yesterday indicated its displeasure with the initiative. An article in the Communist Party newspaper Pravda implied the proposal is part of an effort "to impose on the Arabs unequal separate deals," Washington Post correspondent Dusko Doder reported.

These disparate developments in the fitful Middle East peace process suggest a deterioriation in the foundation of diplomatic nuances on which Reagan administration officials have built their limited optimism since Hussein and PLO Chairman Yasser Arafat announced in Amman Feb. 11 that they had reached an agreement for joint action.

"We're being optimistic about it," President Reagan told reporters last week, adding, "It seems as if some progress has been made."

Although details of the joint plan have not been released officially, several drafts have been published. Announced during the visit of Saudi Arabian King Fahd here last week, and in anticipation of Egyptian President Hosni Mubarak's visit next month, it is a key to the reactivation that moderate Arab states are looking for in the stalled movement toward resolution of the Middle East's complex conflicts.

But diplomatic sources familiar with the official text say that much of its strength lies in its vagueness. The PLO Executive Committee's clarifications yesterday in Tunis appear to deprive the accord of that virtue.

"How can they say they endorse the agreement when they caveat everything important in it?" one State Department official asked privately in reaction to press reports from Tunis. The official said "it's better than if they rejected it and made those points" only because "it keeps the door open to further negotiations."

Specifically, according to diplomatic sources here, hope had been found in the acceptance by Arafat of the phrase "land for peace," as stipulated "in U.N. Security Council resolutions," even though this falls short of specifically accepting Resolution 242 -- which Hussein and the United States have made a central element in their efforts to move the process forward.

In effect, 242 would trade Arab recognition of Israel for Israel's withdrawal from lands it occupied in 1967. But 242 does not guarantee the Palestinians a right to a homeland. Instead, it treats the Palestinians as a refugee problem. As such, the PLO has found it unacceptable.

A PLO statement reported by news agencies yesterday from Tunis after the Executive Committee meeting reaffirmed PLO rejection of 242.

The Hussein-Arafat plan, according to diplomatic sources here, does not specifically demand a Palestinian state, which both Israel and the United States oppose.

But according to reports from Tunis, the clarifications issued by the PLO Executive Committee of its endorsement for the joint action plan said it should be based on achieving "the national rights of the Palestinian people," including the right to return to their homeland, the right to self-determination and to the construction of an "independent state."

Because Israel does not want to negotiate with the PLO as such, the United States has promoted the idea that the Palestinian people may be represented by Hussein as the spokesman for the confederation.

The Hussein-Arafat accord attempts to sidestep that issue, according to diplomatic sources here, by calling for a peace conference attended by the United States, the Soviet Union and the three other permanent members of the U.N. Security Council and all parties to the Middle East conflict.

At the conference, according to these sources, there would be a single Arab delegation in which the Jordanians and Palestinians have coequal representation. If a pan-Arab delegation could not be organized, there would be a joint Jordanian-Palestinian delegation.

The administration found grounds for optimism in the reference to "Palestinian," as opposed to "PLO" representation, according to one State Department official.

But an explanatory memorandum reportedly sent to Jordan by the PLO Executive Committee after yesterday's endorsement of the accord called for "a joint pan-Arab delegation to peace talks, not a bilateral delegation, with the PLO holding a unique and special place in that delegation."