Jan. 20 will be the biggest day in the history of the peanut. It may also be the day Phil Rosen loses a big customer.

Rosen sells nuts. For 4 1/2 years, his Barcelona Nut Company in Takoma Park has supplied nuts to the White House.

Please, no puns. It's just that Richard Nixon and Gerald Ford were excellent customers. As for Jimmy Carter, the identity of his supplier is shrouded in peanut dust.

Asked who will supply nuts to the Carter White House, press spokesman Jerry Doolittle replied, in strict deadpan, "We're still working on the deputy secretary on that one."

Doolittle said Carter's wife, Rosalynn, will be "the key person in the nut dilemma," probably in consultation and concert with the White House chef.

Doolittle was at pains to assure the public, however, that there will "be peanuts in the White House." The President-elect, he said, "has a historical and traditional fondness for peanuts. He eats them out of gratitude, if nothing else."

A lot of people eat peanuts for a lot of reasons, and because of that, Phil Rosen says he will not become an instant pauper if Carter choose someone else. After all, Barcelona grossed more than $1 million in 1976, of which the White House accounted for only about $300, Rosen said.

"The only thing that will hurt," said Rosen, in an interview at his small store and factory at 7328 Carroll Ave., "will be my pride."

Barcelona got the nut contract with the White House in the summer of 1972. The company had actually supplied nuts to every Administration since Eisenhower's but always through a caterer, never directly.

"I got his call one day," Rosen recalled. "Guy said it was the White House calling. I said, "Yes, sir!"

The order was for 15 pounds of mixed, for a state dinner. A few hours later, a limousine arrived. Two men came in and picked up the order. "Cash on the barrelhead," Rosen said. "They always pay that way."

They wouldn't have had to. "They've got great credit, let me put it that way," said Karl Kessler, Barcelona's vice president and Rosen's nephew.

The only change in Barcelona's relations with the White House came when Ford replaced Nixon. The standing order for Ford functions was changed to the deluxe mix - which contains no peanuts.

Political tea-leaf readers would have a good time with that titbit, but not a Ford press spokesman. He said he had no comment, and didn't like nuts of any kind himself.

Rosen understands that he has a major handicap in his wish to keep his flagship customer. His peanuts are all gorwn in the Norfolk and Suffolk, Va., area.

In Georgia, Rosen said, "they actually grow a lower grade Virginia peanut." He buys his Virginias from Virginia, Rosen said, because they taste better and because freight charges are lower.

Barcelona has one big "in" with the Carter folk, however - past performance. "Every Friday during the campaign," Rosen said, "the staff ordered 50 pounds of jumbos in the shell. It was all for the spirit of the thing. I guess they proved their point."

But perhaps Barcelona's biggest edge is that it is the Washington area's only nut wholesaler. There are two others, of about equal size, in Baltimore.