Republican Party Chairman Frank J. Fahrenkopf Jr. was booed here today by NAACP convention delegates after he said blacks were the only voting block in the last presidential election to support a Democratic ticket that was "not just rejected but repudiated by most Americans."

Benjamin L. Hooks, executive director of the NAACP, went to the microphone, stepping between the GOP chief and the audience of more than 3,000, to ask the delegates to stop booing. Earlier in the speech, Fahrenkopf had tried to quiet the hissing by asking the mostly black delegates to "listen to me, please just listen to me."

A tense atmosphere filled the Dallas Convention Center shortly after Fahrenkopf began a recital of presidential election results by voting blocks, hammering at the isolation of blacks.

Fahrenkopf said 62 percent of Italian-Americans voted for President Reagan; 56 percent of Roman Catholics; 57 percent of women; 45 percent of Hispanics; one-third of Jews and 63 percent of Americans over age 60.

He said he was particularly pleased that 60 percent of Americans under 40 voted for the Republican ticket, along with 68 percent of first-time voters.

Continuing the barrage, he said that, if blacks were not counted among labor votes, then Reagan "actually won a majority of the labor vote."

"The Republican Party today is riding on the crest of a rising demographic wave," he said. "However, this victory is not yet complete because of one last statistic. Ronald Reagan and George Bush received only 8 to 11 percent of the black vote . . . ."

At that point a burst of applause broke from the delegates. Fahrenkopf waited. Then, as those applauding began to stand, he asked whether the applause was coming from the "8 or 11 percent" that voted for Reagan.

"Blacks alone, among all demographic groups, cast their votes almost exclusively for the Democratic ticket -- a ticket that was not just rejected but was repudiated by a margin of more than 16 million voters, 49 states and 525 electoral votes," Fahrenkopf said.

The booing began.

"Why did this happen?" Fahrenkopf continued, despite the boos. "Why did blacks isolate themselves from other Americans, including other minorities, in supporting a Democratic ticket that was out of step and out of tune with the overwhelming majority of Americans?"

"Please don't do that," Hooks told the delegates. "We have invited this man, he is our guest . . . . It takes a hell of a lot of courage for him to be here."

Fahrenkopf resumed his speech, saying blacks have yet to integrate the two-party system, in part because they are still tied to Great Society programs.

He said those programs had failed to increase black family income, compared with that of white families, and have led to more single-parent families.

"Obviously we must rethink our Great Society solutions," he said, and suggested that the president's economic proposals, aimed at reducing inflation and interest rates, are the key to progress for blacks and all other Americans.

Fahrenkopf's speech was again met with scattered booing and hissing when he told the delegates that "the Republican Party and the vast majority of Americans oppose preferential treatment for any group."

At a news conference afterward, Fahrenkopf said he was not surprised at his reception: "I know how strong emotions run . . . ."

S.W. Tucker, a Richmond attorney on the NAACP board, said Fahrenkopf had "a tough job."

"What the Republicans do speaks so loudly it's hard to hear what they say," Tucker said.