President Carter began his uphill reelection campaign today with a strong appeal to black voters, his most loyal supporters four years ago but who are showing signs of frustration and disaffection.

Painting GOP presidential nominee Ronald Reagan as a man hostile to black aspirations, Carter said the coming campaign provides the sharpest ideological clash since Barry M. Goldwater challenged Lyndon B. Johnson in 1964.

Carter today sought to allay any feelings that bitterness lingered between him and his defeated Democratic challenger, Sen. Edward M. Kennedy. The president brushed aside any significance to the Massachusetts senator's refusal to give the traditional arms-raised sign of unity when he appeared with Carter Thursday night at the close of the Democratic National Convention.

"It was not easy for him to come to the platform, because he is a fighter and proud person," Carter said. "Kennedy indicated [Thursday] night that we've got to be united because he sees, as I do, that [their] differences are minor."

"I believe it has been one of the most exciting and constructive conventions that we have ever seen," the president said. "It could have been a 1968, where Hubert Humphrey won the nomination but lost the election at the convention."

The importance of the black vote to his reelection campaign was a theme that Carter stressed in remarks to a morning meeting of black delegates and later to the traditional post-convention meeting of the Democratic National Committee.

"One thing that's been significant in my own political career is that I have always been underestimated by our opponents because I have a secret weapon, and that is the black people of this country, who know they have a friend in Jimmy Carter," the president told the black delegates.

Carter was elected in 1976 with more than 90 percent of the black vote, which made the difference in several northern industrial states. That support has now eroded, largely because many blacks blame Carter's economic policies for rising unemployment rates.

Reagan, in an unexpected move, earlier this month began to court black voters, breaking with the longstanding Republican tradition of ceding the support of minorities to the Democrats.

Big city Democratic politicians attending the convention here said Reagan might siphon off some votes of the new, young black middle class. But their greater worry, and clearly Carter's, is that far larger numbers of blacks -- the working class and poor, jobless and increasingly alienated against all politicians -- will stay home election day, Nov. 4.

Carter emphasized to the DNC and the black delegates the importance of voter registration drives.

But his theme was the threat a Reagan presidency would pose for black causes.

"When [Reagan] says anything about state's rights, I don't want any of you to forget that that's a code word [for] discrimination," Carter told black delegates.

"This is a dangerous election, and if you look to the future to what might happen if the Republicans win, then you will be shocked as I have been into going all out to make sure we whip the Republicans as they have never been whipped before."

Of the 481 black delegates here, one in three came pledged to Kennedy.

Because of these conflicting preferences and the participation of many black delegates in the politics of their state delegations and special-interest causes on issues ranging from gay rights to the concerns of organized labor, the well-attended black caucus meetings that were typical of past conventions were not in evidence here.

Nevertheless, there were rump meetings and threatened walkouts by black delegates, inspired largely by the Rev. Jesse Jackson of the Chicago-based Operation PUSH (People United to Save Humanity). Jackson, although not a delegate, was a ubiquitous presence.

He laid out a series of demands in the rump meetings and sessions with administration aides: jobs, better access for blacks to the Oval Office, and more vigorous prosecution by the Justice Department of alleged civil rights violations.

Carter briefly mentioned these concerns in his low-key, seemingly off-the-cuff remarks today to the black delegates.

"One of the things that bothers me is the attempted resurrection of the Ku Klux Klan, but we're going to put them back in the grave and keep them there," he said.

"I have told Drew Days," the assistant attorney general for civil rights, "that any U.S. attorney or anybody in the Justice Department who doesn't agree with what I'm saying is out."

Carter said the delegates need only contact his aides or telephone Rosalyn Carter or the Oval Office if they want to reach him.

Making no mention of his economic policies or his unsuccessful efforts to defeat a platform plank calling for a $12 billion antirecession jobs package, Carter promised action on jobs.

"I know from experience that the most severe, acute hunger in a person's mind is for their life to be meaningful," he said.

The delegates loved it, frequently interrupting him as they stood, applauded and chanted "We want Jimmy, we want Jimmy."

At one point, an emotion-filled, raspy voice cried out in the manner of a black Southern Baptist suddenly gripped by a Sunday sermon. "Speak to us," the man said with feeling.