IT WAS AT a White House Christmas party in 1977 that First Lady Rosalynn Carter, standing beside the holly-decked mantle in the State Dining Room, was first introduced to a 300-pound, self-proclaimed Santa Claus named John Ellsworth.

Ellsworth-felon, forger and government informer on friends and foes alike -- bears little resemblance to St. Nicholas.

But his con-man's eyes can be very merry and his expansive girth can shake like a bowlful of jelly when he tells you -- as he first told Mrs. Carter -- of his love for the little children of the world and his idea he had for circling the globe, dispensing the good things of life to deprived youngsters.

With that spiel, Ellsworth ensnared Mrs. Carter and later her son Chip into lending respectability to the International Children's Appeal, a questionable charity scheme that one participant has since described as "the scam of the century."

Who is John Ellsworth and how has he managed to cause so much trouble for so many people -- including the Carters -- and still ride around Manhattan in a chauffeured baby-blue Lincoln limousine, seeming to have no troubles of his own?

Ellsworth, a high school drop-out, has told people he once drove a taxi. Using the name John James he toured with the Rolling Stones and appeared on camera in the film "Gimme Shelter." w

Ellsworth says he got close to "Candidate Carter" in 1976 by going down to Plains with a camera crew to make a documentary movie for a "black union" he represented at the time. Plans to show the film on television never worked out, he says.

He got close to Rosalynn Carter with a similar ploy, he says. He got permission from the White House to do a documentary movie on the annual Christmas party for diplomats' children.

It was at that part that he first told her about the ICA. He promised to use it to raise money for her favorite charity, the "cities in Schools" program which operates out of the Executive Office Building. ICA raised $7,500 for "Cities in Schools" through one Isaac Hayes concert at Constitution Hall and in total, Ellsworth says, contributed between $60,000 and $70,000 to the project.

In exchange, Ellsworth got prestige money can't buy. He had business cards printed for ICA with Chip Carter's name on them, even though the president's son wasn't working for ICA. He was working for "Cities in Schools."

Ellsworth and a girlfriend attended the Egypt-Iaraeli peace treaty signing at the White House in March, an honor that greatly impressed those of his partners looking to continue expanding their operations in the Middle East.

These days, no one at the White House will answer any questions about Ellsworth or ICA or even return phone calls if they are told he is the subject of inquiry.

John Ellsworth and group of other "independent businessmen" had formed an organization called the "International Children's Appeal in New York, according to an ICA brochure, to "provide the fund-raising mechanisms" for agencies all over the world which wanted to support the United Nations Internatonal Year of the Child.

What Ellsworth didn't tell Mrs. Carter was that the "independent businessmen" associated with him included a number of organized crime figures who had one plan to solicit "donations" from top labor leaders and keep a percentage for themselves, and another plan for ICA to sell toys manufactured by a New York factory that a law enforcement source says is a front for loan-sharking and narcotics operations.

The "independent businessmen" also included: One partner who was a friend of PLO leader Yassar Arafat and had plans for taking over all hashish growing and trafficking out of Lebanon.

One partner who had a scheme to kidnap the shah of Iran and use the ransom money to arm certain Third World countries.

One partner with alleged American CIA connections who actually dealt in arms, with a "catalog" that included a "laser gun that can cut a tree in half."

Partners who were drilling for oil in Ghana, building a hotel in Cairo and claiming Anwar Sadat's son-in-law as a co-investor, negotiating contracts to split $2-million fees 50-50 with Muhammad Ali for a series of personal appearances in various Middle Eastern countries.

Partners who were planning to use Carter administration contacts to produce a $10-million television series about a presidential assistant who drops out and becomes a hitch-hiker.

Millions, possibly even billions, could have been made on the International Children's Appeal. One former associate of Ellsworth's claims that one deal alone was expected to bring $400 million and "four of us were going to each take out $10 million apiece as our share."

Ellsworth admits to having personally "blown $2.8 million" on high-living since he started ICA. And he helped one of his partners wire-transfer another $1.2 million to banks in Beirut.

"But it was our money, made on legitimate business deals," he says. "It wasn't taxpayers' money. So whose business is it?

John Ellsworth -- whose alias in police files around the country include John Jaymes, Clifford J. Ellsworth and Thomas Fiorelia -- did not drop down the White House chimney three Christmases ago.

So how did he get there?

John Ellsworth and his activities for the past 11 years are well known to at least a half dozen U.S. government agencies, including the Secret Service.

By Ellsworth's own account, at least one member of President Carter's personal bodyguard recognized Ellsworth on sight as someone who had once been "reimbursed" $2,500 by the Secret Service for "cooperating" in a counterfeiting investigation.

The agent expressed "surprise" at seeing Ellsworth socializing with the Carters, Ellsworth says, but did nothing more than caution him, to "behave like a good citizen."

John Ellsworth does not like the word "informant." He prefers to describe himself as someone who has "cooperated" in the past with government investigators. He said he has "cooperated" at one time or another with the Secret Service, the Federal Bureau of Investigation, the Internal Revenue Service, the Drug Enforcement Agency, the Central Intelligence Agency and the Organized Crime Strike Force in the Eastern District of New York.

A DEA spokesman refuses to confirm or deny reports that Ellsworth has a DEA informant's file number. But the spokesman, Ted Swift, says there is "not a scintilla of truth" in a claim made by Ellsworth to a reporter recently that the ICA was actually a worldwide narcotics investigation "front" that had taken him and the government "five years to put together" and is still "ongoing."

The DEA did not alert the White House that Ellsworth might become an embarrassment to the Carters, Swift says, "because we didn't know anything about what he was doing."

But somebody in the government must have known and why they did not warn the White House remains a mystery.

The government has been making a lot of cases against Ellsworth's former friends and associates. Sixteen of them were indicted in New York in April on conspiracy charges involving marijuana, cocaine and hashish.

Another four have been arrested on extortion charges. One was "Joey the Priest" Calder of Brooklyn who is a business associate of known organized crime figures.

Ellsworth, who talked with a reporter for eight hours recently, confirmed that he had signed letters of agreement with "Joey the Priest" that would have given him and "his people" a cut from two different deals with ICA involving labor union donations and toys.

"Joey owns a part of this toy company," Ellsworth says. "I meet with him in a Chinese restaurant on 86th Street and he brings his lawyer and he says he can give us a monopoly on selling these stuffed toys to all the street festivals the church runs in New York. He ws in the seminary, you know, that's how he got his name, and he brings me this real priest who guarantees the deal."

The charges against "Joey the Priest" last week involved topless bars and had nothing to do with Ellsworth or the ICA that Ellsworth admits knowing anything about.

Two other Brooklyn mobsters, James Eppolito and his son Jimmy, were shot to death in eary October. Some law enforcement sources have blamed the killings on the fact that the younger Eppolito's involvement with Ellsworth and ICA were attracting too much attention.

Young Eppolito, who had his picture taken here with Rosalynn Carter in April, at a luncheon for "Cities in Schools," was in charge of lining up "contributions" from labor leaders in the AFL-CIO and the teamsters and elsewhere, Ellsworth says.

Ellsworth claims to have had a telephone call after the killings, telling him not to get "too curious" and instructing him to "take care of" Jimmy Eppolito's widow financially.

Ellsworth laughs when told that many of his former employes at ICA are convinced that he himself is in no danger because it is widely believed in certain criminal circles that he "works for the CIA."

"Keep saying that," he says. It's true that he "once took a fall" for the agency, he claims, and they "owe" him. He won't tell the whole story, just dropping the facts that it involved a Russian spy ship that sailed into Galveston to pick up $6-million worth of meat which Ellsworth was supposed to be selling to the Greek government and never did.

Ellsworth also claims to have tipped the CIA to the fact that their station chief was going to be assassinated in Athens six months before it happened.

He also claims to have assisted the Secret Service in preventing an assassination attempt against former prime minister of Turkey at a New York hotel.

A lot of what Ellsworth says about himself is substantiated by reliable government sources and the rest can neither be proved nor disproved. When he came before a federal judge in Brooklyn in 1976 on a charge he later beat, "one or more government agencies" wrote letters in his behalf, saying that he had been of value to them in the past and would be again. The agencies are not identified in court records.

Three former employees of ICA claim Ellsworth bragged he caused Studio 54 its troubles with the federal government. Ellsworth had gotten into a fight with one of the disco's owners, Steve Rubell, when ICA took the place over for a Halloween fund-raiser in 1978.

Ellsworth just smiles and says things about "the wheel coming around" and "all of us getting what's coming to us."

But he still has connections.

Sitting in the Madison Avenue office of one of his lawyers two weeks ago, Ellsworth dialed from memory a lot of his contacts in government, the Secret Service and the Justice Department, talking to them while a reporter listened.

"Can you still dial into that government computer bank, John?" the lawyer asked, explaining to the reporter: "God! How I wish I had been in the collections business when he had that code and access. I mean to tell you this guy could find out anything about anybody from that computer."

Ellsworth explains that he no longer has direct computer access. "there were too many abuses," he says. "The wrong kind of people were using it. . ."