President Carter, ending the self-imposed isolation of his "Rose Garden" campaign, boosted his new secretary of state, Edmund S. Muskie, today and in the process deftly sideswiped Muskie's predecessor, Cyrus R. Vance.

"My hope is that Ed Muskie . . . will play a somewhat different role than the one Secretary Vance played because of a difference in background and temperament and attitude," the president told a questioner at a town meeting at Temple University this afternoon.

"I see Ed Muskie as being a much stronger and more statemanlike senior citizen figure who will be a more evocative spokesman for our nation's policy, not nearly so bogged down in the details of administration of the State Department, perhaps not quite so bogged down in the details of protocol like meeting with and handling the visits of a constant stream of diplomats who come to Washington."

White House press secretary Jody Powell said Carter did not intend to criticize Vance, but that was clearly the thrust of his comments, which came in response to a question about the persistent reports of conflict between the State Department and the president's national security adviser, Zbigniew Brzezinski.

Carter denied, as he has in the past, any serious divisions in the making of administration foreign policy, calling such reports "primarily a creature of the American news media." He said of Brzezinski that he is "kind of feisty, he's aggressive, he's innovative, he puts forth bright ideas, some of which have to be discarded."

The president's indirect references to Vance, whose resignation over the Iran hostage rescue mission Carter has termed "honorable," were perhaps his most cutting observations about any of his cabinet officers, past or present.

Powell later quoted Carter as saying he intended no criticism of the former secretary, and that his comments were intended as part of the process of making the secretary of state more clearly the chief spokesman for American foreign policy.

While foreign policy issues dominated the president's appearances here today, his visit to Philadelphia also marked a turning point in terms of domestic politics.

Except for the brief visit to San Antonio to see the servicemen injured in the Iran rescue mission, it was Carter's first trip out of Washington since Oct. 29 and, as such, a first, tentative step toward open campaigning in the 1980 presidential campaign.

White House officials insisted that it was an official, presidential trip, not a campaign appearance. But this was scoffed at by aides to Sen. Edward M. Kennedy (D-Mass.), Carter's Democratic presidential rival, who pointed out that Philadelphia is the major media market for much of southern New Jersey, which will hold its presidential primary June 3.

For his part, the president chose to make his first out of town appearance in more than six months the occasion for a broad defense of administration foreign policy, beginning with a speech to the World Affairs Council of Philadephia.

The speech bore the unmistakable imprint of Brzezinski's influence as Carter reaffirmed his commitment to arms control and detente but warned the Soviet Union that it "cannot recklessly threaten world peace" and still enjoy "the benefits of cooperation with the West."

The speech was greeted with polite applause, but in the town meeting atmosphere at Temple University the president's appearance provoked friendly and enthusiastic cheers.

Standing on a stage against a background of a huge American flag, Carter fielded questions for an hour and at first showed a few signs of campaign rustiness after the long months of confinement in the White House.

Asked by one questioner why he had now decided to resume travel around the country, the president denied his widely quoted remark of two weeks ago that he considered the Iranian crisis and other issues "more manageable."

Carter asserted that he never said that about the hostage crisis, and said that, in fact, "We are not any closer to getting our hostages back from Iran." But he said "the total complex list of things with which I have to deal with is more manageable," and added, 'I am not at this time involved in that careful detailed planning leading up to [another] imminent rescue mission."

Our domestic issues, Carter held out the promise of a tax cut next year "when the budget is balanced" and predicted better times ahead economically if the nation is patient with his anti-inflation policies.

Throughout the official visit, Carter managed to squeeze in some political fence mending. Arriving here, he was all smiles when greeted by Philadelphia Mayor William J. Green, who supported Kennedy in the Pennsylvania primary.

At the town meeting, the president went out of his way to praise Green as a "dynamic and aggressive and courageous young and new mayor."

Carter narrowly lost the Pennsylvania primary to Kennedy but was badly beaten in Philadelphia and its suburbs. He will need Green's strong support if he is to carry the state in a general election campaign against the Republican nominee.

Carter also demonstrated again the advantages that only a president can bring to a campaign. Asked about the Pentagon decision to transfer a defense contract region from Philadelphia to Georgia, he announced that the decision was being reconsidered and that Philadelphia had a chance to retain the jobs.

"If I were a betting man, I would say that the odds are not against Philadelphia," Carter said.