Former president Richard Nixon was set to buy a $1.8 million apartment on Park Avenue today, but a last-minute court order temporarily blocked the deal.

Jacob M. Kaplan, a 92-year-old philanthropist and business tycoon who lives in the building, brought suit to stop a telephone conference call that would have completed the sale. Nixon, Kaplan reminded fellow tenants in a letter, was "the first president in American history to resign. Except for the pardon subsequently granted to him by his successor, president Gerald Ford, he would have faced criminal proceedings."

Kaplan said the 13-story apartment house at 760 Park Ave., near 72nd Street, is "a quiet, peaceful, secure, unostentatious refuge in this turbulent city. If Nixon were to come in, all this would change . . . There are nutty people around, some who might want to do Nixon harm. Should we be exposed to these possible threats against our lives?"

New York's powerful co-op boards on the Upper East Side are notoriously suspicious of celebrities. The former president was rejected by two apartment houses, on East 72nd and at 817 Fifth Ave., before buying his current 15-room home in Saddle River, N.J., in 1981.

A few months ago, Paul Newman tried to sublet an apartment at 1088 Park Ave. "A male board member blackballed him and all the women were crying their eyes out," according to the son of a resident. Newman reportedly found another Park Avenue apartment with a friendlier board.

Barbra Streisand produced letters of recommendation from Gov. Nelson Rockefeller and Mayor John Lindsay a few years back, but it didn't help her gain admission to 927 Fifth Ave. A board member later remarked, "She'd probably have given a lot of parties."

Kaplan, the man who is trying to block the Nixon purchase, is something of a celebrity himself. A multimillionaire and former owner of Welch's Grape Juice, he once controlled half the Cuban exports of black strap molasses and backed Castro's making of munitions in a sugar mill in the mountains. Later he regretted it, and used his J.M. Kaplan Foundation as a conduit for $1.25 million in CIA funds for a training center in Costa Rica in the 1960s.

Arturo Peralta-Ramos, vice president of the board at 760 Park, said the six-member board will meet face-to-face Tuesday, rather than by conference call. As a result, Kaplan dropped his suit, which claimed the conference call was illegal under co-op rules--a position upheld by New York Supreme Court Justice Wallace Cotton.

"Mr. Kaplan was looking for publicity," said Peralta-Ramos, an investment banker. "He spent three hours in my office ranting on about his hatred of Mr. Nixon."

Peralta-Ramos expects the board to approve Nixon's application for the 12-room apartment. "Mr. Nixon is a perfectly nice gentleman, not a rambunctious man," he said. "He doesn't go looking for publicity. He happens to be one of those people it comes to."

Nixon reportedly wants to move into the city because of his wife's failing health.

Nicholas Ruwe, Nixon's spokesman, approached Peralta-Ramos about the apartment. Ruwe's father, Peralta-Ramos said, "is a great friend. We're bird hunters."

Ruwe was unavailable for comment, and was reportedly quail shooting in Texas.

Celebrities aren't the only group spurned by the co-ops. Diplomats are nongrata, because they have legal immunity and cannot be pursued for nonpayment of fees. Third World envoys are particularly unwelcome, as Prince Saud al-Faisal, Saudi Arabia's foreign minister found out in 1977 when a Park Avenue co-op rejected his bid for an 18-room apartment.

"There's considerable tension in the Mideast and, who knows, we might have had pickets out in front of the building, maybe bombs, bullets, firebombs, poison in the water," one tenant said at the time.

For years many Upper East Side co-ops did not accept Jews, Irish Catholics or blacks. River House, a bastion of wealth and status at 435 East 52nd, turned down Gloria Vanderbilt in 1980. The heiress, who had applied to buy a $1.1-million duplex, filed suit claiming she was rejected because of her friendship with Bobby Short, the celebrated black pianist.

Brokers say there are still perhaps a dozen buildings where Jews, blacks, Asians or Arabs are unwelcome, although it is against the law to discriminate on the basis of race. Discrimination is virtually impossible to prove, however, since co-op boards can turn people down on the basis of personality. Legally, co-ops are not required to give reasons for rejecting applicants.

Regarding Jews, said Maxine Dubin, a successful agent here, "In some buildings, unless you're a Morgenthau or a Guggenheim, you haven't a Chinaman's chance."

Another agent said she tried to place a Japanese woman, daughter of the owner of a major electronics firm, in an Upper Park Avenue apartment last month. "I was told not even to present her to the board," the agent said.

Homosexuals are unwelcome in many buildings, and musicians are often rejected because they "make noise." Pianist Misha Dichter was turned down at 101 Central Park West a few years ago, but found an apartment on the same street.

The principal discrimination, however, is merely financial. A nine-room apartment that sold for $125,000 five years ago, sells for up to $1 million today, according Mikki Gold, an agent with Phyliss Koch Real Estate. She said the housing squeeze in Manhattan is so tight, and rental buildings are so scarce, particularly in the better neighborhoods, that co-ops are requiring 50 percent cash and in some cases 100 percent cash, even on apartments that cost more than $1 million.