President Carter brought his bandwagon out of mothballs today and discovered too late that it was rusty from disuse -- slow in starting, unsure in handling, accident-prone on even the easiest of courses.

There was the president, standing before a "town meeting" in his first public appearance outside Washington in half a year, handling an opening question for which he was well-prepared, when suddenly -- much to the shock and quick discomfort of his aides -- his apparent deep-felt frustration with Cyrus R. Vance just started tumbling out.

By the time the president was launching into his last answer of the day, his press secretary, Jody Powell, had appeared behind the wooden camera stand to explain to reporters that the president really had not intended to criticize Vance, even if it sounded that way.

Eventually Carter steadied his course and wound up with a generally effective political performance, in the view of those in his audience. But there was a series of presidential miscues in Carter's maiden outing of the 1980 campaign season, and the most startling came after Carter was asked the expected question about Washington's latest eternal triangle: Vance-Brzezinski-Muskie.

All Carter had to do was state for public consumption his assertion (albeit a shaky one) that there never had been policy differences between his advisers -- that life in the inner circle had been "a practically unanimous consensus."

But, to the surprise of his aides, not to mention his audience, the president could not brake in time and he was rolling on to say that, in comparison to Vance, he believed Muskie would be "much stronger . . . more statesmanlike . . . a more evocative spokesman for our nation's policy . . . not nearly so bogged down in details of administration of the State Department."

For good measure, he labeled Muskie a "senior citizen figure" and branded Zbigniew Brzezinski a Polish American . . . [who] is feisty, he's aggressive, he's innovative, he puts forth bright ideas, some of which have to be discarded. . . .

An hour later, as Carter was plunging through his closing answer of enthusiasm and uplift, reporters clustering around Powell were asking him for clarifications about various things his boss had said faster than it was possible to answer.

Muskie a "senior citizen"? "He meant senior adviser," Powell said. "It's just what you folks would call a type, that's all."

At one point, the president had declared: "I don't say this bragging because I haven't done enough yet -- [but] I've appointed more federal judges in the three years that I've been in office than all the other presidents put together since this nation began." The president meant more judges who were black, Powell said.

And when the president said that it was more difficult for the United States to know where the hostages in Iran are, "with our hostages now moved to different countries," he meant to say cities, according to Powell.

The president also offered a clarification of his own concerning his explanation last week that he could end his Rose Garden seclusion and see America once again because our problems are now "manageable." He said today that he did not mean to imply that the problem of hostages in Iran is now more "manageable" -- although that clearly was part of the context in which he spoke in his original comment back in Washington.

The president's visit received saturation coverage on the local television and radio stations in Philadelphia -- which blankets the southern half of New Jersey, a crucial primary state in the big June 3 multistate windup to the 1980 campaign. One station, KYW, devoted its entire afternoon to breathless minute-by-minute coverage of the president's visit.

Yet the Carter-Mondale campaign did not pay for the trip -- but Edward M. Kennedy and Ronald Reagan and, in fact, all of America's taxpayers did. m

The visit was essentially a copy of the one that had been laid on last November, only to be canceled by Carter after the hostages were seized. Thus, White House aides said, it was presidential and not political in nature, even though the New Jersey primary is at hand. The town meeting audience was selected from those who had clipped coupons in newspapers in November and had gotten tickets to attend at that time, Carter aides said.

After his initial rustiness in this first outing, the president managed to establish a pace and rapport with his audience that characterized his more successful campaign ventures of the past.

He found ways to work into his answers some of the populist phrases that serve as touchstones of his last presidential campaign. There were references to those who live in the "wealthy suburbs" and to those who buy food at lower prices and then "put it in a fancy deep freeze."

He gave hope to a woman who worried that a Philadelphia defense contract was about to be switched to a Georgia site, came armed with a statistic that he said showed that 98 percent of those who lost their jobs when the federal government closed its Frankford Arsenal here had either found new jobs or had retired.

And -- in the finest tradition of a politician consolidating for the fall -- Carter even had words of profuse praise for the local mayor, William Green, whose endorsement of Kennedy may well have provided the margin that brought about Carter's razor-thin defeat in last month's Pennsylvania primary.

Epilogue: There were hints, even at the outset, that this might not be the best of campaign days for Carter. The White House had chosen to bring the press corps from Washington by bus. On the outskirts of town, the buses suddenly were pulled into a circle in a parking lot on Broad Street, and over the walky-talky of one White House aide came the voice of another, apparently oblivious to the Philadelphia skyline plainly in sight.

"Which way on Broad Street -- north or south?" asked the lead aide. For just a moment, the White House had lost Philadelphia.

They were, as one bus rider noted, just a little rusty.