At least one Roosevelt won't be on hand Thursday at Ronald Reagan's big deal for the New Deal, a White House luncheon marking the 100th anniversary of Franklin Delano Roosevelt's birth.

Smarting over Reagan's failure to appoint a centennial year commission, Franklin Delano Roosevelt Jr. said yesterday that though his brothers, Elliott and James, have accepted the president's invitation, he has made other plans.

"I'm surprised that the president did not appoint a centennial commission, yet suddenly he invites the entire family to the White House," the youngest Roosevelt son said by telephone from his Clove Crook Farm in Poughquag, N.Y. "I wonder how many pictures will be taken to give the impression to the public that he is on intimate terms with the family?"

Roosevelt said that while he believes Reagan's admiration of his father is "quite sincere -- he did vote for him three or four times," he also thinks such sentiments have to be viewed in the light of some of Reagan's actions.

"My father was a great conservationist, but the Reagan administration is undoing steps that my father's administration took 40 to 50 years ago. And the threat to Social Security concerns me very seriously," said Roosevelt.

Equally irked at Congress for what he called its "lack of leadership" in organizing centennial year observances, Roosevelt said he wasn't boycotting the White House.

"I'll be representing the family," he said. Today, he'll be in New York City at an FDR ceremony Mayor Edward Koch is hosting, on the weekend he'll be at Hyde Park. And apparently that's only the beginning, considering the number of invitations he's had from all over the country.

"These are celebrations that are purely spontaneous," he said, "which is far more significant and important to my father's memory."

Two others staying away from the Reagan lunch will be Robert Nathan, who was chairman of FDR's planning committee of the War Production Board, and author Joseph Lash, whose latest book, "Love, Eleanor," comes out this spring. Nathan says his absence is for health reasons and that he is resting up in Key West for hip surgery he'll undergo in March. Lash, also in Key West, says he's not protesting. "I just don't want to go."

Even so, not since Jimmy Carter lived there have Democrats been so welcome at the White House. Politicos expected include House Speaker Tip O'Neill, Democratic National Committee chairman Charles Manatt, Senators Edward M. Kennedy (D-Mass.) and Daniel Patrick Moynihan (D-N.Y.).

Others among the more than 200 guests will be Winston Churchill's grandson and namesake (Pamela Harriman's son by her marriage to Randolph Churchill; she and former New York governor W. Averell Harriman are also on the list); New York Gov. and Mrs. Hugh Carey; Diana Hopkins Halstead, daughter of the late Harry Hopkins; journalists Alistair Cooke, Jack Anderson, David Brinkley, Joseph Alsop, Al Lewis and Chalmers Roberts; historians Henry Steele Commager, James MacGregor Burns and Arthur Schlesinger Jr. (who heads Carey's FDR centennial commission in New York state); FDR's secretary Grace Tully; and Clifton and Margaret Truman Daniel.