Sen. Edward M. Kennedy (D-Mass.), in a speech aimed at moving his party and his political image toward the center, today challenged Democrats to "reinvigorate" themselves by learning to "do more with less" and leading "a country, not a collection of divided and contending groups."

Weighing into the debate over the party's future for the first time since the Democratic election debacle of last November, Kennedy said Democrats can neither "blame the voters" for their defeat nor pretend "that the essential problem last year was President Reagan's television performance."

He accused Democrats last year of losing "the feeling of hope, the spirit of change" that had marked the party in the past, and called for a reexamination of the party's positions in light of the realities of the 1980s.

"We cannot and should not depend on higher tax revenues to roll in and redeem every costly program," Kennedy said. "Rather, those of us who care about domestic progress must do more with less."

He said Democrats must show "the courage to discard" outdated programs, adding that, "The mere existence of a program is no excuse for its perpetuation, whether it is a welfare plan or a weapons system."

Reacting to criticism that the Democratic Party has become a collection of warring constituencies, Kennedy said fellow politicians must learn to say "no" to special interests and in the process unburden the party of its special-interest image.

"As Democrats, we must understand that there is a difference between being a party that cares about labor and being a labor party," he said at a symposium on the presidency of his slain brother, John F. Kennedy. "There is a difference between being a party that cares about women and being the women's party. And we can and we must be a party that cares about minorities without becoming a minority party. We are citizens first and constituencies second."

Today's speech is certain to add to the speculation that Kennedy, who unsuccessfully sought the presidency in 1980 and declined to run in 1984, is preparing for the race in 1988, and it was a clear reminder to other potential candidates that he intends to be a major force in the debate over the party's future.

One Kennedy aide said earlier that, whatever the direction of that debate, it is essential for Kennedy to be seen as being in the forefront of it. As part of that effort, Kennedy today strongly endorsed tax simplification, calling the current tax system a "code of Babel."

Kennedy press secretary Bob Mann called the speech "a revisiting rather than a change" in the Massachusetts senator's political philosophy. Kennedy sought to highlight his role in airline and trucking deregulation and criminal justice reform to remind voters that he has been more than a liberal champion of the poor and a creature of big government.

Often portrayed as a defender of the Great Society, Kennedy said today that the party must break the "mythology that all the Democrats have to offer is more programs at higher cost for lesser returns." He said the nation "cannot face our actual problems with an ideology that is always pro-government -- or always antigovernment."

He said that while the party should not break with the past to abandon its commitment to the poor, he is "totally convinced that the indispensible condition of compassion is economic growth."

Generally harsh in his criticisms of Reagan and the Republicans, Kennedy paid the president one generous compliment. "Whatever the wisdom of his policy, Ronald Reagan has restored the presidency as a vigorous, purposeful instrument of national leadership," Kennedy said.

In positioning himself in the Democratic debate, Kennedy appeared to have his sights set on potential 1988 rivals. Without mentioning Sen. Gary Hart (D-Colo.) and Hart's campaign of "new ideas," Kennedy said the "test of an idea is not whether it is new or old but whether it is right or wrong."

And he singled out for criticism one new idea -- means tests for Social Security and Medicare beneficiaries -- being advanced by Arizona Gov. Bruce Babbitt (D). Babbitt currently is exploring a presidential candidacy.

Kennedy accused Reagan of having "a reckless military budget," "a weapons policy" instead of a foreign policy, of embracing "repression" in Latin America and of being seen as aiding apartheid in South Africa. He said Republicans are not caught in the "grip of ideology" and have become "defenders of their own status quo."

He also warned Democrats to "avoid the temptation to think that by changing labels or manipulating images, we can trick people into believing that we, too, have seen the conservative light."

"We must offer new ideas," he added, "but they must be more than retreads of the reactionary nostrums of this day."