The presidential candidates grabbed the headlines at the New Jersey Democratic state convention here this week, but it was the oratory of the younger generation that kept the delegates talking long after the results of Tuesday's nonbinding straw poll had been announced.

Sen. Joseph R. Biden Jr. (D-Del.), in a loving rebuke to a political party that he says he believes has gone astray, brought the 1,500 delegates to their feet when he vowed that he and other younger Democrats are prepared to rededicate themselves to the values that made the Democratic Party dominant over the last 50 years.

"The experts believe that, like the Democratic Party itself, the less than 40-year-old voters are prepared to sell their souls for some security, real or illusory," Biden said. "They have misjudged us. Just because our political heroes were murdered does not mean that the dream does not still live, buried deep in our broken hearts."

It was an oratorical high note in a speech filled with soaring rhetoric, and the delegates came up out of their seats--not with the hurrahs that are a conditioned response to applause lines, but with the kind of raw emotional release that swept like a tidal wave through the huge casino ballroom and lingered long after Biden had left.

Biden's speech was all the more interesting because watching on the podium was New Jersey Sen. Bill Bradley, another talented young Democrat who also may have ambitions. Bradley, a former Rhodes Scholar and professional basketball star, has distinguished himself in the Senate on a variety of issues, and has become the party's leader in formulating a new taxation program.

His speech to the delegates was a solid and sober Democratic platform for the future.

"The courage to seek change when things begin to go stale is the key to the future of our party and our nation," Bradley said. "The promise of America is not comfort, but freedom, not a numbing self-preoccupation, but an opportunity to compete on reasonably equal footing. America was never based on an equality of results, but on the persistent commitment to unlock our land's potential and then to share its bounty fairly with all the American people."

But if Bradley offered Democratic bedrock, Biden sought a moral uplift, linking his political awakening to the civil rights movement and the campaigns of two early Kennedys.

But he confessed, "At age 40, my attention has been diverted and I have deferred action on my vision for America as a consequence of being overwhelmed by the pleadings of special interests."

The Democratic Party has gone astray, he said, "by failing to remember what got us this far and how we got here--moral indignation, decent instincts, a sense of shared sacrifice and mutal responsiblity, and a set of national priorities that emphasized what we had in common."

He castigated President Reagan, but he particularly blamed his own party for giving the Republicans an opening. And he said the Democrats can win back the White House and their previous power "only if we stand openly and unbendingly on those moral issues that are at the core of our soul."

Biden called for leaders who would tell farmers in Iowa that feeding hungry Americans is "more important than selling grain to the Russians," who would tell teachers' unions that "competency is as important to the revitalization of education as higher pay and smaller classsrooms," or who would tell women's groups "that restoration of a compassionate government and a sane nuclear policy are as important as freedom of choice" and the Equal Rights Amendment.

"To my generation has now come the challenge," Biden said. "In the days to come, we will be tested on whether we have the moral courage, the realism, the idealism, the tenacity and the ability to sacrifice some of the current comfort to invest in the future.

"I believe that this generation will rise to the challenge," he said. "These . . . people will march to the right drummer."