With senators gathered around and the cameras whirring, a masked convicted car thief demonstrated yesterday just how easy it is to steal even the newest-model automobiles.

With a ruler-shaped thin metal device he called a "Slim Jim," the criminal, whose name was not revealed for his own safety, quickly slid it down the side of the window of a 1979 Chevrolet Malibu. In less than 15 seconds, he had unlocked the front door.

Then using a GM force tool," a device available to auto repossessors and salvage yards, he wrenched out the ignition switch from the steering column. A matter of another 60 seconds or so. To drive the car away, he would have slipped in a new ignition switch to which he had the key.

"At the time of my conviction," said the convict, who is still serving a five-year sentence for conspiracy to transport a stolen motor vehicle, "I could steal almost any American-made car in less than 90 seconds, and could steal most in 40 to 50 seconds."

Although he said he's been out of the car-theft business for 19 months, he showed he hadn't lost his touch. With a variety of tools, he rapidly broke into and started a half-dozen exhibits representing the doors and ignitions of Ford, Chrysler and General Motors vehicles.

The convict, referred to yesterday only as "John Smith," said his arrest resulted "from my involvement in a major vehicle theft ring consisting of 40 to 45 individuals" operating in nine Western states and Mexico. "I have personally stolen over 700 American-made cars in my life," he told the Senate Permanent Subcommittee on Investigations.

"Smith" said he started stealing cars because it was "so easy" and there was big money in it. "It seemed to me that anyone with common sense who did not take stupid risks could get away with stealing cars. He said he "lived a very high life style" and that some in his group earned close to $200,000 in 1977.

"smith" testified at the opening session of Senate hearings this week and next on professional automobile theft and "chop-shops." Chop-shop is the term used when thieves strip a stolen vehicle to sell its parts to repair shops.

According to a committee fact sheet, the black market in auto parts is profitable because new replacement parts are so costly and often there are weeks-long delays in delivery. It now costs $23,000 to separately purchase parts for a 1978 auto costing $5,100, according to the fact sheet. As for delays, the report said "A phone call to a salvage yard dealing with chop-shop operators can cut the delivery time to one day."

Sen. Charles Percy (R-Ill.) called vehicle theft "a national growth industry that reaches directly into the pockets of millions of Americans." The Justice Department, he said, estimates that it is costing the American consumer $4 billion annually, including more costly insurance premiums.

Organized crime, he said "has allegedly muscled into the chop-shop business and has taken control." Sens. Percy and Joseph Bidden (D-Del.) have introduced legislation to require indentification numbers on all major body parts and to improve locking mechanisms on doors, trunks and ignition systems.

"Smith," whose face was covered by a gray wool ski mask, said he turned to crime when he was working as a line mechanic in a Southern California dealership and rebuilding salvaged autos on his own.

In 1966, two men sold him a set of Corvette bucket seats for $150, when the going rate at salvage yards was $350. He said he realized they must be stolen. After that, he got more deeply involved. In 1977, he was arrested on a search warrant and a former associate testified against him.

He said he always threw away any personal belongings he found in a car he stole.

"Even a $40 or $50 sweater?" asked Percy. "You got rid of it?"

"Yes," said the convict."It would have been stupid to have a $40 sweater get in in trouble for a $4,000 profit."