Sen. Gary Hart (D-Colo.), in a speech brimming with the zestful rhetoric of John F. Kennedy, today launched his bid for the 1984 Democratic presidential nomination by exhorting his fellow Americans to "once more become a bold, adventurous and pioneering nation."

At 45, Hart is the youngest of the Democratic presidential hopefuls, and he staked his claim as lead spokesman for a new generation of politicians, forged by Vietnam, Watergate, energy crises and the arms race, who are ready to "break out of the deadlock of old systems and old doctrines." He called the policies of the Reagan administration "economically wasteful and morally wrong," but said he also rejected traditional liberal approaches that are "frozen in the ice of ineffective programs."

Rather, he advocated an economic policy that would provide incentives for investment and entrepreneurship, and would have as its centerpiece the education of young Americans for the technologies of the future.

In military affairs he called for a policy of investing in conventional forces and small, maneuverable weapons over such big-ticket expenditures as the MX missile and B1 bomber.

He also pledged that in the opening months of his administration he would go to Geneva and negotiate with the Soviets for a mutual freeze on nuclear weapons production, to be followed by a negotiated reduction in the nuclear arsenals of both countries.

Hart promised that his campaign would be dedicated to breaking the grip of "narrow, special-interest government in Washington."

To symbolize that determination, he said he will accept no contributions from political action committees. In his 1980 reelection campaign Hart received 21 percent of his funds from PACs, about the norm for Senate incumbents that year.

His campaign organization also announced that he had raised enough money to qualify for federal campaign matching funds. To qualify, he had to raise at least $100,000 in contributions of $250 or less, totaling at least $5,000 in each of at least 20 states.

In keeping with Hart's fondness for high technology, it was beamed around the country by satellite through C-Span of Washington, D.C., and the Cable News Network of Atlanta. His campaign committee also arranged to fly tapes to Iowa and New Hampshire for cable channels in those key early states.

Hart is the second Democrat to officially enter the race, along with Sen. Alan Cranston (D-Calif.) They are to be joined next week by former vice president Walter F. Mondale and former Florida governor Reubin Askew.

Hart, son of a Kansas dust bowl farmer, entered the national political scene as campaign manager for George McGovern's 1972 presidential race. But it was Kennedy who first attracted him to politics as a student volunteer in 1960, and it was Kennedy to whom Hart made his oratorical bows today.

He mimicked Kennedy's phrasing and staccato delivery, and updated one of his more memorable lines:

"The American people are ready to respond to a president who does not promise what government can do for each separate group--but who asks what all of us can do for our country and our common good."

In a post-speech news conference, Hart made clear that he does not count the traditional constituent blocs of the Democratic Party--labor, blacks, ethnics--among the "narrow special interests" against which he is campaigning. But he also made clear that he would not always kowtow to their legislative agenda.

Hart is the only Democratic hopeful in the Senate who has not endorsed the domestic-content automobile bill favored by organized labor. His aides acknowledged this could be a problem with labor, which hopes to endorse a Democratic presidential contender this fall. Hart said yesterday that he will actively seek labor's endorsement.

There are other problems looming over his candidacy. While he has earned high marks in the last year for innovative, sometimes iconoclastic policy papers, his aides admit that, at the moment, his candidacy lacks a galvanizing theme.

"We have no Vietnam," said political director William Shore.

And, a Gallup Poll last month on 16 possible Democratic candidates showed Hart 13th in name recognition, with only 22 percent of the sample group having heard of him.