President Reagan yesterday signaled his willingness to reduce defense spending next year, as long as it does not "compromise national security."

The president told a group of reporters that he was "completely in line" with White House chief of staff Donald T. Regan's comments Sunday, opening the way to a possible compromise with Senate Republicans on a lower defense budget than the president has sought.

Asked about Regan's suggestion that reductions might be achieved by reducing waste or "stretching out" procurement of munitions, food and fuel, the president said, "That door is always open. The one thing, as I say, we will not do is compromise national security."

The comments reflected a shift of tone, if not necessarily of substance, from the harder line Reagan took in a Saturday radio talk, in which he emphasized that "vital weapons systems, either conventional or strategic, must not be touched" by congressional budget-cutters.

Senate Budget Committee Chairman Pete V. Domenici (R-N.M.) said he welcomed the statements. "I think they indicate some willingness to negotiate," he said. The Republican majority on the Budget Committee had approved about $11 billion less for fiscal 1986 than Reagan had requested.

In the hour-long interview, Reagan also:

* Said a summit conference with new Soviet leader Mikhail Gorbachev would be welcome in September, or any other time Gorbachev suggested, as a forum for discussing both countries' suspicions of nuclear treaty violations, among other topics, but said "there have been no signals" of the Soviets wanting such a meeting.

* Said there has been "some pretty sizable progress" in opening Japanese markets to American goods, and suggested that if there is frustration with current negotiations, "it certainly won't be the fault" of Japanese Prime Minister Yasuhiro Nakasone.

* Praised Vice President Bush as "the best" in history and said he was fully qualified to be president, because "there isn't that much difference between the two jobs."

* And suggested that, in light of a recent Supreme Court decision, Congress should abandon all limits on campaign contributions "as long as [they] are reported . . . out in the open."

In softening his previously stated resistance to defense cutbacks, Reagan said the the administration had "volunteered" savings of $16 billion from its earlier estimates and was ready to look at more savings "if this can be done without endangering national security."

But he threw doubt on suggestions of some senators that significant savings could be achieved in personnel costs. The president said he approved the increase in civilian employment in the Defense Department, because that is less costly than assigning uniformed personnel to noncombat duties.

Reagan said he would be happy to see Gorbachev if the Soviet chief comes to New York for the September session of the United Nations, but added that "it's only proper that we allow him to set the schedule . . . ." Asked about Soviet violations of past nuclear-arms treaties, Reagan said that "we have to recognize . . . they've got suspicions that they think are legitimate with regard to our intent. And maybe sitting face to face, we can work on the thing of convincing them . . . and find ways where we can by deed prove what our intentions are."

The president pointedly declined to join other administration and congressional figures in criticizing Japanese resistance to opening their home market to American telecommunications equipment, threatening current negotiations in Tokyo. He also suggested that private American assistance to the anti-Sandinista forces in Nicaragua was in keeping with "this country's tradition of private giving . . . whether it's this or whether it's disasters from earthquake, flood or . . . famine."

On domestic politics, Reagan said it was too soon for him "to join the speculation on 1988," but praised Bush, who was sitting two seats away, in lavish terms. "I don't think there's ever been a vice president that has been as much involved at the highest level in our policy-making and our decisions [as] George, or that there has been a better vice president than he has. He's been the best."

In commenting on last week's Supreme Court decision knocking down limits on group spending on behalf of federal candidates, the president expanded on the court majority's view by saying that, whether it is in the form of a contribution or direct spending, "to me, this is a part of free speech." No limits should apply, he said, "as long as it is reported."