President Reagan last night opened the door to a possible meeting with a Jordanian-Palestinian delegation as the first step toward Arab-Israeli peace talks, but he ruled out any direct United States role in such negotiations.

In his 29th nationally televised news conference, Reagan said "we're willing to meet" with a joint delegation, as Egyptian President Hosni Mubarak urged him to do in a meeting here March 12, and which King Hussein of Jordan suggested last weekend as a way to revive the Middle East peace process.

The administration had not been enthusiastic about a meeting with a joint delegation when Mubarak suggested it as part of a three-stage peace plan aimed at bringing about direct talks between the Arabs and the Israelis.

Last night Reagan reiterated his view that the United States would not meet with any delegation that includes representatives of the Palestine Liberation Orgaanization (PLO), but said he is certain that other Palestinian representatives could be found.

In the second formal news conference of his second term, Reagan also:

* Said he opposes extension of a federal program giving up to 14 weeks of extra unemployment benefits to more than 300,000 jobless workers who have exhausted regular state payments.

* Held firm against compromise on Social Security and defense spending, two key budget issues separating the White House and Senate Republicans, who are to meet with Reagan today.

* Said he does not think the new Soviet leader, Mikhail Gorbachev, has "rebuffed" Reagan's invitation for a summit meeting in the United States, and predicted that the prospects are good for such a summit.

* Said he has decided not to visit a Nazi concentration camp on his trip to West Germany in May to commemorate VE Day because he feels "very strongly" that "instead of reawakening the memories and so forth, and the passions of the time," there should be a "celebration of an end of an era and the coming into what has now been some 40 years of peace for us."

* Applauded Senate approval of $1.5 billion to build an additional 21 MX missiles and said that, unless a tradition of "bipartisan unity on defense" is upheld by House approval of the same request next week, "there's little prospect of success" in the current Geneva arms negotiations.

On the Middle East, Reagan insisted that he has not put in place a new policy of "disengagement" from the region. Mubarak charged in a speech here March 13 that the United States was taking "almost a defeatist approach" to the Middle East peace process and settlement of the Palestinian issue.

"I believe it's a misapprehension that President Mubarak left disappointed," Reagan said. "He made no requests. He told us what he was doing; and certainly we complimented him highly upon what he's doing. I think it's great."

Mubarak said when he was in Washington that he had asked Reagan to invite a joint Jordanian-Palestinian delegation to meet here to explore the chances for a settlement.

Reagan said the United States "did not want to participate" in direct Arab-Israeli talks. As for a joint delegation, "we did have to make it clear that we couldn't meet if it was the PLO" because "they still refuse to recognize U.N. Resolutions 242 and 338, and they refuse to agree or admit that Israel has a right to exist as a nation."

A Feb. 11 pact between Hussein and PLO chairman Yasser Arafat backed all U.N. Security Council resolutions on the Middle East but failed to mention Resolution 242 specifically. That resolution, adopted after the 1967 Mideast war, calls for Israel to withdraw from occupied Arab lands in return for a guarantee of peace and secure borders. But it makes no mention of the Palestinians as more than a "refugee problem."

Hussein, in an interview with The New York Times published last Sunday, urged Reagan to meet with a joint delegation but ruled out any talks that do not include the PLO.

"We're willing to meet with a joint group -- Palestinian and Jordanian," Reagan said last night. "But at the moment, not the PLO because . . . we have not had any statement from them that they do recognize Israel and that they will recognize 242 and so forth. But there are many Palestinians who don't feel that they're represented by the PLO. And any delegation of them -- for example, many of those who are living and holding local offices on the West Bank."

Reagan used the opening statement of his 32-minute news conference to lobby for House approval of MX missile funds. The House vote next week, he said, "will answer the question of whether we stand united at Geneva or whether America will face the Soviet Union as a nation divided over the most fundamental questions of her national security."

The president said he is going to "wait" to deal with a problem that could be caused later this year with the launch of a Trident II submarine with missiles that appear to exceed strategic arms limitation talks (SALT II) limits on the total number of sea-based and land-based multiple-warhead missiles each side can deploy. Reagan said, "I can assure you we're not going to do anything that's going to undercut the negotiations" with the Soviets in Geneva. As for the submarine, "We're going to wait and deal with that problem when we come to that point . . . . "

Asked about criticism of his proposed Strategic Defense Initiative (SDI), or "Star Wars" missile defense system, from British Foreign Secretary Geoffrey Howe, and a response this week from Assistant Defense Secretary Richard N. Perle, Reagan said he had not seen their speeches and wanted to see "what the exact words were about it."

"I do know that we have the support of Prime Minister Margaret Thatcher and, therefore, the English government in our research for the Strategic Defense Initiative, and so I'm satisfied with that," he said.

Asked if he was giving any thought to recognizing the rebels fighting the Sandinista government of Nicaragua as a "government in exile," Reagan said, "No, we haven't thought about that at all." Congress has shut off covert U.S. aid to the "contras" and Reagan is expected to launch a major political push to seek to win $14 million from Congress for them this spring.

The $14 million "isn't the issue," Reagan said. "The issue is the United States is trying to help people who had a communist tyranny imposed on them by force, deception and fraud, and either we continue with that tradition which has always been ours, or we give that up entirely, and I don't think we should give that up. I think our position is clear."

Reagan also was asked about his decision not to visit the Nazi death camp Dachau. He will be in West Germany from May 2 to May 4 for an economic summit. Reagan aides decided last month that he would not take part in ceremonies in West Germany May 8 marking the Allied triumph over the Nazis 40 years ago, but instead will address the European Parliament in Strasbourg, France.

Asked why he is not visiting a Nazi death camp, Reagan said that instead of "reawakening the memories and so forth and the passions of the time, that maybe we should observe this day as the day when 40 years ago, peace began and friendship; because now we find ourselves allied and friends of the countries that we once fought against. And that we -- it can almost be a celebration of the end of an era and the coming into what has now been some 40 years of peace for us."

He added that he felt that the German people "have a feeling, and a guilt feeling that's been imposed on them, and I just think it's unnecessary. I think they should be recognized for the democracy that they've created and the democratic principles they now espouse."

Questioned about reports that he would seek to discipline congressional Republicans who are not loyal to the White House on key votes by not raising money for them in the 1986 elections, Reagan said, "No. I've never done that." He recalled that he has always tried to follow the "11th commandment" that "Thou shall not speak ill of another Republican." Reagan has, however, criticized Republicans he has run against, including an incumbent president, Gerald R. Ford, from whom he tried unsuccessfully to wrest the nomination in 1976.

On the defense budget, Reagan said the administration had already cut its five-year projections by $150 billion and that his fiscal 1985 defense request is $16 billion less than what had been projected for 1985 by President Jimmy Carter.

"So we think we have made sizable cuts already," Reagan said.

The president did not mention that Carter's projections were based on the assumption of continued high inflation. The fact that inflation plummeted during Reagan's first term allowed him to cut billions from the defense budget without reducing actual purchases of weapons.

This "inflation bonus" has been documented by Reagan's aides.

According to a study by the Congressional Research Service, if the inflation factor is adjusted for the actual levels in Reagan's first term, 1981-84, then Reagan added 10.2 percent, or about $90 billion, to defense spending, more than Carter projected for the same period.

Several journalists wore bright red dresses and several wore red ties to the news conference because of a long-held suspicion that the president is more likely to recognize a reporter wearing Nancy's Reagan's favorite color.

The Wall Street Journal noted this in a story yesterday, and Reagan opened the questioning by telling Helen Thomas of United Press International, who was wearing brilliant red, that "I know that Nancy upstairs would die -- she's watching on television -- if I didn't call on you in that pretty red dress."