WASHINGTON builder Nathan Landow -- who lost an expected ambassadorship early in the Carter Administration after publicity about his ties to major underworld figures -- is in line to become a U.S. representative to the United Nations General Assembly at this fall's 35th session.

His nomination, coming at a time when Cngress is getting ready to go into a Labor Day weekend adjornment, could result in a "recess appointment" where he would serve without confirmation on an interim basis until the Senate Foreign Relations Committee gets around to studying the matter.

However, one source on Capitol Hill said last week that the Landow nomination will be carefully scrutinized once Congress reconvenes.

Senators are expected to ask Landow about his connections to three men:

Washington gambling kingpin Joe Nesline, who acted as a "consultant" on a proposed hotel and casino project which Landow had intended at one point to build in Atlantic City.

Edward Cellini, an internationally known gambling figure who is the brother of former Meyer Lansky associate Dino Cellini. Landow was involved with Cellini on another proposed hotel-casino project that had been proposed, but never built, on the island of St. Maarten.

Anthony Plate, the Gambino organized crime family's alleged representative in Miami who was indicted last year in New York on racketeering charges involving murder and loan sharking. A spokesman for the Justice Department's Organized Crime unit says that Plate is now missing and presumed dead.

Florida law enforcement authorities reported to other police agencies in October 1973, that Plate was believed to have a 25 percent investment in Quaker Masonry Inc. Landow, listed in 1972 corporate records as a vice president and director of Quaker. Landow told a reporter in 1978 that he had between $120,000 and $160,000 invested in that company.

According to law enforcement sources, Landow has been questioned at least twice during the past year about his relationship to Nesline. Landow was summoned to the U.S. Attorney's office in the District of Columbia for quizzing last November and a month later New Jersey State Police were asking similar questions.

Landow's nomination has not yet been submitted by the White House for congressional approval. But he confirmed last week that he expects to get the appointment and is scheduled to be briefed by the State Department for the job on Sept. 9 and 10.

He has made the necessary financial disclosures, he said, and has undergone a security check and been cleared.

Landow, despite embarrassing publicity about business relationships with various shadowy figures, is a man who moves in the highest circles of the Carter Administration.

He is a close personal friend of Hamilton Jordan. Landow's weekend retreat on the Eastern Shore, where Jordan himself often goes for rest and relaxation, was used by top campaign officials last August for a strategy session.

Landow, Maryland finance director for the Carter-Mondale campaign, ran the $500-a-plate kickoff dinner here last December.

He accompanied campaign chairman Robert Strauss to China on a trade mission.

Landow, who had very little involvement in Democratic politics until Carter's 1976 campaign, was in line to become U.S. ambassador to the Netherlands shortly before papers found during an FBI raid of Nesline's apartment in January 1978, linked the two men.

A search warrant for Nesline's apartment in a Landow-owned high-rise in Bethesda, The Promenade, netted plans for a 1,000-room, $85 million Atlantic City hotel and casino project that Landow had intended to build with Nesline's guidance.

Landow sought unsuccessfully at the time to keep a story about the raid from being printed until after his ambassadorial appointment to the Netherlands had been announced by the White House.

The announcement was "imminent," he said. But after the story ran, it was never made.