I have a tentative plan to retire in the year 2000." said Rep. Claude Pepper (D-Fla.), who will be 80 in September and is the oldest member of the House. "But I may change my mind."

Looking like he can continue to run for the next 20 years without even breathing hard. Pepper was the beneficiary and guest of honor at a $250-a-plate fund-raising dinner last night in the home of another New Deal veteran, attorney Thomas J. Corcoran. "Tom was one of the right-hand men to President Roosevelt; he was the one who got things done," Pepper said, thanking his host for his hopitality.

Now a fiery advocate of the rights of the aged, as chairman of the House Select Committee on Aging, Pepper also got things done in the Roosevelt years. His advocacy of strong preparation against Hitler, including Lend Lease and conscription, was so effective that he was hanged in effigy on the Capitol grounds in 1940. He still has the dummy, which bore a placard reading "Claude Benedict Arnold Pepper," and he said last night. "It is one of my most treasured possessions. I plan to leave it to the University of Florida along with my papers."

"I'm the notorious thespian from wicked New Your City," said Sarah Pepper Wills, the congressman's kid sister who is a mere 68. She was referring to one of the most famous speeches in one of the most bitter senatorial campaigns in modern history, when Pepper, branded as a leading liberal and sometimes as "Red Pepper," was defeated by George Smathers in 1950. In a speech which relied on his audience's unfamiliarity with big words, Smathers said that Pepper "is known all over Washington as a shameless extrovert. Not only that, but this man is reliably reported to practice nepotism with his sister-in-law, and he has a sister who was once a thespian in wicked New York. Worst of all, it is an established fact that Mr. Pepper, before his marriage, practiced celibacy."

His sister admitted that she was, in fact, a thespian: "I minored in drama and majored in English, and I've been on the stage from the time I was 2 years old. I sang the blues with Sophie Tucker, and she told me that I was the last of the red-hot mommas." Ten she went over to Corcoran's piano and began playing "Darktown Strutters' Ball." A crowd gathered around the piano, began singing along in medley of Irish songs, but let her solo on "A Good Man Nowadays Is Hard to Find" -- which was, in fact, red-hot.

The air in Corcoran's backyard garden was warm with ond memories. "I first met this man nearly 60 years ago on the steps of Harvard Law School," said Corcoran. "How long we have lived to frustrate our enemies."

"The senator was very kind to my husband and me when we first came to Washington in 1941," said Rep. Lindy Boggs (D-La.). "Not only was he very nice to us socially, but he served as my husband's mentor." Nearly everyone calls Pepper "sentor," although he has been in the House since 1962, and a testimonial letter from President Carter, read at the party, began, "Dear Senator."

Boggs brought a ceramic statue of a female donkey for Corcoran, explaining that "he has a female elephant in his front hall because his mother was a Republican, so I thought he should have a female donkey, too. He told me that anyone who doesn't follow his father's politics and his mother's religion is either an ingrate or an abused child." Pepper agreed to pat the donkey for a photograph and immediately started the latest controversy in a long controversial life.

"Did I see the senator patting an ass?" asked a bystander.

"He told me he has practiced celibacy with everyone except his wife," said another.