NEW YORK, JUNE 8 -- Sen. Sam Nunn (D-Ga.), edging closer to a decision about whether to seek his party's presidential nomination, gave a foreign policy talk here today that left a roomful of skeptical liberal Democrats singing his praises.

"It was a very impressive performance," said John V. Connorton Jr., a lawyer who was Gary Hart's 1984 New York cochairman. "I was particularly taken with the way he answered the question about contra funding. This was an audience that opposes it, and he gave a very forthright explanation of why he supports it."

"His story about Mario the ballplayer was terrific -- it really showed he has a feel for local politics," said Gordon Stewart, a veteran Democratic activist. A prominent Democratic fund-raiser said Nunn "could raise a lot of money in this town."

Nunn spoke at the invitation of New York Gov. Mario M. Cuomo and the state Democratic Party in one of their ongoing series of issues forums, this one attracting an audience of several hundred.

The Armed Services Committee chairman opened by publicly kidding his host. He told a tale of young "Mike" Cuomo's days as a minor-leaguer in Georgia 35 years ago, weaving make-believe baseball history ("Cuomo never struck out in the South") with beloved baseball aphorisms ("It ain't over 'til it's over") to suggest that, denials notwithstanding, his host could yet wind up as a presidential candidate in 1988.

When Nunn was asked by reporters afterward whether he planned to seek the presidency, he said: "I'm one step behind Gov. Cuomo." Cuomo was actually standing a step behind Nunn at the time, and, sensing the interview was heading toward a place he didn't want to be, quietly slipped off the stage. Nunn pressed on. "I haven't closed the door," he said, adding that he hopes to make a decision by the fall.

Nunn's surprising turn at humor overlapped into the substantive portion of his presentation. He described the chain of events beginning with the arms sales to Iran, leading to Iran's enemy Kuwait seeking military protection for its shipping in the Persian Gulf from the Soviet Union, leading to the United States also offering such protection and then discussing the possibility of a strike at Iranian missile sites, and concluded, deadpan: "I sure hope we don't make the moderates in Iran mad about all this."

He said he believed Iran "ought to be kept in doubt" about whether the United States would launch a preemptive strike against Iran if it deploys Silkworm missiles.

In response to a question from the audience, Nunn said he supports continued funding for the contras fighting the government of Nicaragua. "I don't think a military solution is possible. But I do not think a diplomatic solution is possible without military leverage," he said.

If Nunn were to enter the presidential race, he would be the only Democratic contender to support contra funding. Another prominent Democratic leader from the South thought to harbor national ambitions -- former Virginia governor Charles S. Robb -- is scheduled to speak to the Foreign Policy Association here Tuesday and he, too, will call for continued funding.

Should Nunn become a candidate, his problems with New York liberals -- and many other Democrats -- would more likely come not on foreign policy, but on the domestic front. He has consistently opposed abortion funding, opposed the Equal Rights Amendment and favored many of President Reagan's economic and budget policies.