Ricardo Nunez was on the campaign trail in Miami's "Little Havana," pitching to his fellow Cuban exiles in stacatto Spanish that the time has finally come to unseat the area's venerable Democratic Rep. Claude Pepper.

"Your vote for me will be a vote for Hispanics," the bulky Republican real-estate investor pledged, about midway through a campaign stemwinder.

The Republican primary runoff in the newly redrawn congressional district today matches Nunez against Manuel Iglesias, another Cuban-born American. The winner will take on Pepper in the general election.

Pepper, 82, the oldest member of Congress, was unopposed for the Democratic nomination to an 11th consecutive term. Because of his long years tending to constituents, particularly the many elderly in Miami Beach, the conventional political wisdom is that either Nunez or Iglesias will have a difficult job retiring him -- this time.

As the two Hispanic candidates indicate, however, the contest already has accentuated the changing ethnic makeup of Pepper's longtime South Florida fiefdom and the growing ambition of Miami's swelling Latin population to send one of its own to Congress.

"This district will be represented by a Latin; there is no doubt about it," predicted Don Hayes, the manager of Iglesias' campaign. "The question is whether it will be beginning next January or two years from now. I believe it can happen now."

The political potential of Latin voters is perhaps nowhere more striking than in Florida, where 470,000 of the nation's 800,000 Cubans lived even before the Mariel boatlift brought in 180,000.

In the past month, the White House has welcomed leaders of major Hispanic organizations on two occasions. Some wealthy Miami Cubans recently formed a political action group to help finance campaigns around the country against congressmen they regard as soft on Fidel Castro.

Hispanics accounted for 29 percent of the vote in Pepper's district two years ago, with Jews representing 25 percent and blacks 18.5 percent. Others, including "Anglos," made up the rest.

The Democratic-controlled state legislature then redrew the district, slicing off some Cuban areas and adding the southern tip of Miami Beach -- and its elderly Jewish voters who have turned out to back Pepper over the years.

Hayes still calculates that, partly because of a recent Republican voter registration drive, Hispanics make up slightly more than half the district's registered voters.

Cuban voters here generally tend to be Republican because of conservatives' harder line against Castro. President Reagan, strongly identified as anti-Castro and anticommunist, carried the district with 80 percent.

"Foreign policy is really domestic policy in Dade County," said Nunez's campaign manager, Tony Cotarelo, referring to the interests of the Latin and Jewish voters.

Pepper, with his 20 years in the House and 15 previous years in the Senate, knows that as well as anybody.

He has supported the Reagan administration's Radio Marti plan to broadcast news unfavorable to Castro to Cuba. Leader of the House delegation to the international Interparliamentary Union, he boycotted a regular session last year because it was in Cuba.

As head of the U.S. delegation to the Interparliamentary Union, Pepper also had the opportunity to impress his Jewish constituents. He led American legislators out of the recent session in Rome when it was addressed by Yasser Arafat of the Palestine Liberation Organization.

Pepper's constituent casework includes benefits for the elderly; a Cuban immigrant works full time sorting out immigration problems for families who have been helped--in their native Spanish--in getting desperate relatives into the country.

Edgardo V. Caturla, a lawyer who specializes in immigration cases and the son of a prominent immigrant leader, says he and many other Latins support Pepper because "there's always been a perception that you could go to Mr. Pepper's office."

"I'm a Republican, but I'll vote for Pepper," he added. "If we were choosing between two reasonably equally qualified candidates and one was Latin, I of course would vote for the Latin. But there is a lot of support for Pepper because people feel that because of his long years of service in Washington, he ought to be more effective than a newcomer, even a Latin."