Obviously their motives are different but President Carter is doing for sweaters today what Lana Turner did for them in the 1940s. Probably not since the hey days of that Sweater Girl have Americans been so aware of people wearing sweaters.

Carter comes by them honestly. Casual, comfortable clothes are part of the Southern apparel style that Carter prefers. he wore sweaters frequently while campaigning, and as President-elect was photographed in one while conferring with his Cabinet-to-be on St. Simon's Island.

Carter is believed to be the first American President to make on official appearance before the nation wearing a sweater. President Kennedy wore informal clothes in Hyannis, and President johnson had his boot-and-Stetson Pedernales clothes. President's Ford and Nixon donned sweaters for the golf course, but none of these leaders wore them as visibly on the job as President Carter didi Wednesday night.

It's not likely that Carter was wearing a sweater merely for the warmth it provided but rather to underscore the informality of his fireside chat and to put across his message of energy conservation. Unlike FDR, who sat beside a fake fire in the Diplomatic Reception Room for his broadcast talks, Carter sat next to a roaring blaze in the White House Library. According to United Press International, aides advised him to change from the business suit he wore at 9 p.m. when he signed emergency energy legislation, into more casual attire, including the sweater, for his 10 p.m. television appearance.

Others in the White House are using sweaters to counter the cold temperatures. Presidential assistant Midge Costanza, the former assistant mayor of Rochester, N.Y., regards the current cold spell as Washington's way of offering Rochester-type welcome.

She has been bundling up with woolly socks under boots and sweaters under her pantsuits. "It doesn't look any different from what I would wear at home," she said.

It's different for Ellen Egan, assistant to Jack Watson. She's been wearing turtlenecks and crew-neck seaters daily. "These are not the kind of clothes I would think of wearing to an office," said Egan, "but it's just plain cold here.

Mrs. Carter is layering sweaters under jumpers or wearing them over skirts and blouses, her aides report. And Chip Carter has turned in his jacket for a pullover worn over shirt and tie.

Not since Pat Boone wore a cardigan sweater with full sleeves that became adapted as a golf sweater, or Andy Williams wore V-neck pullovers to give him a fatherly image, has so much attention been paid to one man's sweater.

"Imagine television cameras concentrated on one sweater for 30 minutes. It's bound to have an influence," syas an optimistic Jack Schultz, senior vice president of Bloomingdale's.

It's not the specific style of Carter's sweater that is likely to have an impact, however. He was wearing a V-neck wool cardigan, sometimes called a "grandfather sweater" since it sells best as a Christmas gift for older men.

Sweaters are, in fact, no warmer than jackets. But now that many men have adopted the fashion of three-piece suits for business, the wearing of a sweater offers an appropriate alternatives for informality and comfort.

Says designer Calvin Klein, "Women understand this concept of comfortable clothes far better than men, who insist on wearing strict business, suits. Maybe this will help change that." Klein, often wears sweaters in place of vest under his jackets.

Designer Ralph Lauren agrees. "Carter wore a sweater to establish his own identity. It should have an impact greater than a movie star on an athlete. It could change the etiquette of clothes."

Carter's penchant for sweaters isn't about to have any immediate impact on sweater sales in Washington or elsewhere. Those sweater styles which weren't snapped up in post-Christmas markdowns were cleared out of stores, along with gloves, warm coats and long johns, during the recent cold spell.

"If only all this could have happened to help us before Christmas," said Bloomingdale Schultz.