President Reagan, facing a potentially fatal blow to his Middle East peace initiative, insisted yesterday that he still is "very hopeful" that Jordan's King Hussein will join talks with Israel, and said he already is in personal contact with Hussein and other Arab leaders to keep the U.S. proposals on track.

Reagan, who spoke to reporters when he returned to the White House after a weekend at Camp David, refused to accept as anything more than a temporary setback the announcement that Hussein was breaking off talks with Palestine Liberation Organization leader Yasser Arafat. Those talks were aimed at winning PLO approval for Hussein to join the peace process, and the Jordanian announcement said the king will not enter the process independently.

Reagan, blaming the failure of the Hussein-Arafat negotiations on "some radical elements of the PLO," said he had been in touch yesterday with Hussein and King Fahd of Saudi Arabia and planned to communicate later in the day with other Arab heads of state.

Secretary of State George P. Shultz and his wife spent the weekend at Camp David with the Reagans. It had been billed as a social occasion, but U.S. officials said privately that the president and Shultz had kept in close touch with Mideast events and were studying various steps to save the Reagan initiative from collapse.

The initiative, worked out by Shultz and announced by Reagan in a nationally televised speech last Sept. 1, calls for expanded negotiations to give the Israeli-occupied West Bank and Gaza Strip eventual independence "in association with Jordan." Israel has rejected the idea, but the administration has been working on the assumption that if Hussein agreed to negotiate, the Israelis eventually could be won over by guarantees of security and recognition from the Arab world.

Since Hussein is crucial to further progress on the U.S. plan, the administration has emphasized trying to help him win PLO approval to negotiate on behalf of the Palestinian inhabitants of the occupied territories. On Friday, the administration said publicly that if Hussein joined the peace talks, the United States would put pressure on Israel to freeze the establishment of Jewish settlements in the territories.

In his brief remarks yesterday, Reagan said: "Some radical elements of the PLO have introduced changes in the proposals that have been made . . . . Those proposals are unacceptable to King Hussein; they're unacceptable to King Fahd; they're unacceptable to me. And I have been in touch with King Fahd and King Hussein and am going to be in touch with other Arab leaders about this."

In response to questions, the president conceded that the Jordanian announcement is "an impediment in our search for peace." But he insisted that he remains "very hopeful because we're all in agreement about the other proposals that have been made . . . . I have their assurance that they want to proceed with what we've been doing. We've made great progress so far, and King Hussein has made great progress."

However, Reagan refused to give details of the proposals to which he referred or to indicate what moves he plans now. He said, "I can't take any questions, because I can't deal in specifics while I still have other heads of state that I want to communicate with."

Despite the president's confident attitude, it was clear that unless something can be done quickly to reverse Hussein's decision, the U.S. initiative will collapse. If the United States cannot produce Hussein at the bargaining table, it would seem to have no justification for arguing to Israeli Prime Minister Menachem Begin that the Arab countries are ready to accept the existence of the Jewish state and that he should be more flexible about the future status of the occupied territories.

U.S. officials said yesterday that they do not know what strategy Reagan and Shultz will adopt now. The most likely step, they noted, probably will involve stepped-up efforts to convince Fahd and other moderate Arab leaders, who privately favor exploring the American plan further, to use their financial support and other leverage to force the PLO's various factions into an agreement with Hussein.

However, the influence of the moderate Arabs has been countered repeatedly by radical Arab states like Syria, which has encouraged its allies among the PLO's various factions to oppose any deal with Hussein.