Will Hussein become 'Lone Ranger' of Reagan's Mideast peace plan?

King Hussein of Jordan remains intensely interested in joining peace negotiations with the United States and Israel - despite the lack of encouraging signs from the meeting of the Palestine Liberation Organization's parliament-in-exile in Algiers.

The King, say observers in Amman, is far out in front of most of his advisers in his continued search for a way to enter peace negotiations based on President Reagan's initiative of last Sept. 1.

''He is the 'Lone Ranger' in this endeavor,'' said one source in Amman.

However, these sources add, no one can be certain what the King's final decision will be.

''The King has not yet made up his mind,'' says one well-informed source. ''He is a very intuitive man, and at a certain moment he will decide, based on that intuition.''

While the final communique from Algiers is not yet certain, it is clear that the King will not get the ''green light'' he had sought in order to announce by March 1 his readiness for joining talks with the US and Israel. The King would lead a Jordanian delegation also containing non-PLO West Bank Palestinians.

The King needs such a mandate to legitimize bypassing the Arab League's designation of the PLO as sole representative of the Palestinians - including those on the West Bank and Gaza.

However, the March deadline seems already to have been abandoned in Amman, though sources here continue to insist time is short. They say the next few weeks will be a time of watchful waiting to see:

* If there is progress in negotiations in Lebanon on Israeli troop withdrawals, which Jordan views as a test of US credibility as a mediator.

* If PLO chief Yasser Arafat is given leeway by the Palestine National Council (PNC) to give the King a green light later on.

Several factors will influence the King's decision, which Jordanian sources now expect by the end of March (though some caution against further deadlines).

Foremost is the King's belief that the security and future of the ''East Bank'' - the remaining portion of his kingdom after Israel captured the West Bank in the 1967 Arab-Israeli war - is in deep jeopardy if the peace process fails.

''The King isn't in this to get back the West Bank. He's doing it to eliminate the danger to the East Bank,'' says one informed Jordanian source.

The King is said not to be overly optimistic about the chances of the peace process. ''There is a big difference between the action of (the late President Anwar) Sadat in joining peace negotiations and Jordan doing so,'' says a Jordanian official. ''In the case of Sadat, Israel was favorable. In the case of Jordan, based on the Reagan proposals, Israel is not.'' (Israel opposes the Reagan proposals because they call for the return of West Bank land in exchange for peace.)

But, the official adds, ''If the peace process fails we want the Americans to place the blame on Israel, not us.''

Sources here say the King believes a wave of radicalism from the left or the religious right will sweep the area in the next few years if the momentum toward peace through negotiations fails.

He is also said to be deeply worried that Israel might seek to unseat him to make good the assertion of former Defense Minister Ariel Sharon and other senior Israeli officials that the ''East Bank'' (Jordan) is the Palestinian state.

Moreover, he fears that if there is no negotiated settlement to the West Bank's future, the ''East Bank'' might be inundated by West Bank Palestinians, driven out either by force or by unbearable conditions under Israeli occupation.

It is in this context that the King has been evaluating the US assurances, which are conditioned on his joining the peace process.

The King has in his possession two secret letters from President Reagan. The letters' existence is widely discussed in Washington and the Middle East but they are not acknowledged by either Jordanian or US officials.

The Monitor has learned that the first letter summarizes US negotiating positions - basically the positions in the Reagan peace proposals but in tougher language. These include commitments in principle to:

* The return of territory (on the West Bank and Gaza Strip) for peace - a necessary reassurance given Israel's public position that it will not give up the West Bank or Gaza. (However, contrary to rumors here and in the West Bank, the Reagan proposal does not guarantee that 97 percent of West Bank land would be returned. No percentage is mentioned.)

* Negotiations over the future status of Jerusalem.

* A sense of urgency on the time of the transition period leading to negotiations on the final status of the West Bank and Gaza. This should be ''as soon as possible,'' implying sooner than the maximum of five years set out by the Camp David accords. This does not contradict the accords, which use the same phrase and leave open the possibility of a shorter transition time. However, a Jordanian request that the US agree to specify a three-month limit for the period leading up to the transitional phase and a one-year maximum for the transition did not receive a favorable response.

* A strong pledge to work for a full freeze on Jewish settlement in the occupied territories once King Hussein has indicated readiness to join the negotiating process. While US diplomatic sources will not confirm this commitment, they note a public statement by US special envoy to the Middle East Philip C. Habib in which Habib said it is unimaginable to believe King Hussein would sit down at the negotiating table without a settlement freeze.

The second letter indicates that progress on the peace front will facilitate American consideration of Jordanian weapons requests. Specific weapons or amounts are not mentioned. However, the Jordanians are known to be interested in F-16 fighter planes and mobile Hawk anti-aircraft missile batteries. The letter also commits the US to help maintain Jordan's security and stability.

Progress in peace negotiations in Lebanon is not mentioned in either letter. However, such progress remains a key factor in the King's final decision.

The King is said to have originally told the US he did not wish to make the Palestinian question a hostage to solution of the Lebanon problem. But Western diplomats say the linkage between the two issues has become more pronounced in recent weeks.

Jordanian officials say it would be very difficult for the King even to announce willingness to join talks - let alone sit at the table - without a prior agreement on Israeli troop withdrawals from Lebanon.

The US administration believes something less than a signed agreement - though still significant progress - might be sufficient proof of US credibility for the King at least to make his announcement. But the final, perhaps pivotal, consideration of the King will be how much of a green light he decides he needs from the PLO. The King had wanted a clear-cut mandate from the PNC.

''King Hussein is vulnerable unless he has the PLO to share the responsibility,'' says one Western diplomat. ''If the PLO gives a private blessing, they can then wait to see what happens; and if they don't like it, (they can) put all the blame on him.''

Observers here expect that after the PNC meeting there will be a period of intense consultations between the King and PLO chairman Arafat. If the PNC gives Mr. Arafat sufficient leeway, this could permit negotiated guarantees that would satisfy the King.