An interesting piece of political theater took place here Saturday night, starring Sen. Edward M. Kennedy and former vice president Mondale, the two old liberal Democratic rivals.

It was only the second time the two had appeared together since Kennedy began his unsuccessful challenge for the Democratic presidential nomination against Mondale's old boss, Jimmy Carter. And the air was rife with rumors of backstage maneuvering over such weighty matters as who would speak first.

As it turned out, the program for the annual Hubert H. Humphrey Day Dinner listed Kennedy as the first "featured speaker" and Mondale as the second. But the way each interpeted his role before the crowd of 4,000 was more revealing than the order they spoke or anything either said.

Kennedy played the part of the injured crusader, the last and truest keeper of the liberal faith. Mondale played a far more subdued part, that of the grateful favorite son returned home to offer thanks. While Kennedy was strident, Mondale was mellow, almost apologetic.

The evening's humor was also instructive. Kennedy, alluding to "some talk of a feud between us," ribbed Mondale for abandoning liberal issues during the 1980 campaign and of already gearing up for 1984.

He said Mondale, then the vice president, telephoned him at one point early last year and said, "There aren't enough liberal issues to go around, so you can have them all." Mondale, he added later, is so eager to see the Chicago law firm he now works for grow that "he has already set up branch offices in Iowa and New Hampshire."

Mondale, a former Minnesota senator and state attorney general, picked up the same theme when he was presented a suitcase that supposedly had been "found abandoned in a Holiday Inn the fall of 1974," a reference to his early decision to drop out of the 1976 presidential race.

"I do recognize this luggage. Recently, I've been in several of those motels," Mondale said. "They've been redecorated and they look a lot better. mI think I'll be seeing a lot more of them."

Mondale has maintained a low profile during the first months of the Reagan administration. Three lectures he gave at colleges in the Twin cities area were so uncontroversial they barely received mention in local newspapers.

Meanwhile, Kennedy, according to aides, has become increasingly frustrated over the failure of Democrats to rise to the defense of party-sponsored programs President Reagan is trying to cut or eliminate. Kennedy has stressed this theme in recent appearances in Maine and Massachusetts. But aides said his speech Saturday night before a Democratic-Farmer-Labor Party dinner was his most comprehensive defense of liberalism since Reagan's election.

"I'm convinced that the answer to the problems of the Democratic Party is not for us to pretend that somehow we are similar to the Republicans," he declared. "In 1981, we cannot permit the gains of a generation to be set aside by the agents of special privilege. We must not permit our principles to be swept aside by the negativism of the New Right -- or by those who dare call themselves the Moral Majority."

Repeatedly interrupted by applause, Kennedy lambasted the Reagan administration for advocating military involvement in El Salvador, selling F15 airplanes to Saudi Arabia and abandoning detente and human rights.

He claimed that deregulation and supply-side economics are actually Democratic Party ideas, dating back to the days of his brother, John F. Kennedy. "If Kemp-Roth were in fact a prescription drug instead of a tax bill, it would not be allowed anywhere near the national marketplace," Kennedy said.

He also criticized Democrats who suggest "we should not resist the conservative tide," saying, "Now is not the time for silence, delay and ambiguity."

Mondale took a much more low-key approach, spending most of his brief speech thinking Minnesota Democrats for "sticking with me" as the rest of the nation swept Reagan into office. "I'd rather lose with Minnesota than win without Minnesota," he said.

The Minnesota DFL -- the lparty that produced Mondale, former vice president Humphrey and former presidential canidate Eugene J. McCarthy -- has long been considered a bastion of liberalism. Severely splintered in 1978, it was one of the first state Democratic parties to register major defeats when it lost two Senate seats and the governorship to Republicans. With the governorship and one Senate seat up in 1982, the state promises to be one of the most intriguing battlegrounds for the future of both parties once again.

Party officials said Saturday night's dinner raised more than $200,000. Those in attendance gave both Kennedy and Mondale high marks.

St. Paul's popular mayor, Paul Latimer, said Kennedy "said some things that needed to be said." Mondale's speech was a good contrast "from the standpoint of theater and that's what politics is," the mayor added. "It would have been the dumbest thing in the world for him to try to out-Kennedy Kennedy."