For Robert Sowa, it was the ultimate used-car dealer story. There he was in the Carter White House Wednesday night, dining on Supreme of Royal Squab at a state dinner for the president of Romania.

The White House guest list identified him as "Robert Sowa, Manchester, N.H., state representative . . . "

The only trouble is that he is not now nor never was a member of the New Hampshire legislature. The only Robert Sowa that folks in Manchester know is a used-car dealer who used to be an alderman form Ward 6 until his conviction last year for bilking on insurance company of $3,825 over a 1970 over a 1970 Chevrolet Corvette.

Sowa reported the car stolen from his used-car lot on Manchester's Queen City Avenue in July 1976 "when indicted for "theft by deception," a felony, and false reporting of a crime, a misdemeanor, early last year.

At first, there was "talk that he was framed, but we had him cold," recalled Hillsborough County Attorney Raymond Cloutier. Sowa pleaded guilty to both counts in New Hampshire Superior Court on March 3 last year.

"It was a negotiated plea," Cloutier said. "part of the consideration was that he resign as adlerman, which he did," Cloutier added.

Cloutier said he was puzzled by Sowa's presence at the White House amid the elegant china, the plush drapes, the music of pianist George Shearing.

The prosecutor wondered why, above all, Sowa was invited to a dinner for the president of Romania. "I thought he was polish," he said.

Sowa, who runs a used-car dealership in Manchester under a new name, could not be reached for comment, but an old friend, state Rep. Chris Spirou, the Demoncratic minority leader in the New Hampshire House, thought he had the answer.

"Bob was an early supporter of the president," Spirou recalled. "As a matter of fact, he gave Carter his snow boots when he came up here something to the effect that Carter had given him a standing invitation to come and get his boots back. Maybe this was the first time Bob was able to do it."

White House press secretary Jody Powell, who says he borrowed a pair of galoshes from Sowa during the 1976 New Hampshire primary campaign but didn't know whether candidate Carter had, offered another explanation.

"He (Sowa) did help us in the campaign up there, but then - I don't remember exactly what happened - there was a falling out," Powell said. Then, last February on the second anniversary of the New Hampshire primary, the White House staged a party for veterans of that campaign.

Sowa "didn't get invited," Powell said. "He apparently wrote someone up here, and said, 'Look, I know we had some differences, but I really helped you guys. I didn't get invited to the party, but I'd really like to come.'

"So he got an invitation." Powell said. I guess somebody thought it might make for a past injustice."

So how did an ousted alderman get on the White House guest list as a "state representive"? Powell deflected the question with a wisecrack.

"It was a plot to deceive the American public that he was really a full-fledged state representative when he was really just an alderman," the White House press secretary quipped. "Obviously, no one here had any knowledge of his troubles with the authorities."

By all accounts, Sowa was an up-and-coming politician when the Corvette caper laid him low. "He was [manchester newspaper publisher William] Loeb's favorite boy," Cloutier said. "He was a Democrat but he used to get all the publicity. He was a real right-wing alderman. An extreme conservative."

In fact, the 32-year-old car dealer still managed to run second in the September 1976 primary for the Governor's Council seat from the Manchester area despite initial publicity about the alleg insurance fraud. Authorities said Sowa paid $450 to $500 for the fire-damaged Corvette in question, then had it towed out of town while reporting it stolen. Besides being forced to resign as alderman, he drew a suspended one-year sentence, was fined $1,000 and ordered to make restitution to the insurance company.

At the White House Powell professed ignorance of any findings the Secret Service might have made about those invited to Wednesday's festivities. But he said the nature of Sowa's difficulties would hardly have made him a security risk.

The Secret Service makes routine checks on guests to state dinners. However, a criminal record, unless it involved a crime of violence, would not prevent a guest from attending.