No one can say that Nancy Reagan didn't eat a bite at the National Federation of Republican Women's lunch yesterday.

She did eat. One bite. Just one, daintilly chewing it 32 times, while reporters counted sotto voce: "29 . . . 30 . . .31 . . ."

"I once spent almost every minute of an entire day with her in St. Louis," said one Missouri reporter. "And all that I could see that she ate all day were two seedless green grapes. Just two. And she chewed both of them 32 times, too." That is, reportedly, an old weight-watching trick.

Mrs. Reagan and Betty ford were supposed to share double billing at the head table as former and would-be future first ladies. But Elizabeth Taylor Warner timed her arrival so that, if she didn't upstage them, she managed to divide the crowd's attention and adulation into a three-way split.

Other Senate wives showed up early, as they were supposed to, getting their moments of glory out of the way with the hordes of television and camera crews before Nancy Reagan and Betty Ford made their entrace.

But Warner didn't arrive at Table No. 4 until the last minute, so that photographers and reporters and autograph-hungry matrons were forced to go from Nancy to Betty to Liz, as if they were all equally important.

"She looks like a young Ethel Merman," said one woman as she sized up Warner, with her teased bouffant and bangs.

To strangers, Warner looked plump and matronly in a silk dress printed in red, white and blue carnations.But at a table of Virginians (including Gov. Dalton's wife) who got hugs all around from Warner, what they were saying behind the actress' back when she walked away was "how much weight she's lost . . . can you believe it!"

There was a lot of talk at other tables about whether or not "Liz's nose was out of joint" the night before when she sat with Nancy Reagan at the convention in the Regan box. It is widely belived among Republican wives that Warner has hopes of being first lady herself one day, or at least second lady.

She did not look happy on Tuesday night, sitting in her tent-like muumuu beside the beautifully groomed, 108-pound Mrs. Reagan, who is years older but looks years younger.

Politically, one-time actress Nancy Davis is now a bigger star than Elizabeth Taylor, and Taylor doesn't seem comfortable cast in a supporting role.

Betty Ford on the other hand, seems relaxed and happy to have someone else take more of the spolight.

She ate every bite of her lunch, and there were a few tense moments for some onlookers as she sipped what looked like a bloody mary. Then waitresses began serving other tables, and Mrs. Ford's friends relaxed when they discovered the drink was gazpacho in a glass.

Sen. and Mrs. Strom Thurmond were telling their admirers that they had sent their four children off to Vacation Bible School while they were busy at the convention.

Anna Chennault, wearing a lemon-colored lame jacket, was bawling out a local society columnist who had said she looked like "a china doll" at another party this week, wearing pink lace and pink jade jewelry.

Sen. John Tower's wife, sister of international arms merchant Sam Cummings, confirmed reports that her brother has recently been back in the United States.

Cummings, a former CIA agent who now holds Irish citizenship and lives in Monaco while running his global Interarmco, was back "for the first time in years," Mrs. Tower said, "about two months ago."

Mrs. Reagan's "Group" of special friends had been supposed to lunch today with Mrs. Robert Evans in Grosse Pointe. But Nancy Reagan wanted them with her, so they took a table right where she could see them.

The Republican National Committee is going to have to find $550 in the budget to pay for flowers for Nancy Reagan's suite.

Friends had sent her a half-dozen expensive and exotic floral arrangements when she arrived. But the Secret Service threw them away without her ever seeing them. All she found in her room were the cards which had accompanied the bouquets.

She insisted that a duplicate of each arrangement be sent for a small private party the Reagans were giving. The hotel florist agreed, as long as someone pays his bill twice.

There are a lot of ladies of the evening cruising hotel lobbies in Detroit, as there are at every political convention.

But the blond in a red dress looking for an elevator to the Renaissance Club the other night wasn't one of them.

Rep. Margaret Heckier (R-Mass) tried to get directions from a white-suited elderly gentleman who misunderstood. He took her arm and wouldn't take no for an answer.

"You don't understand," she protested. "I'm a member of the House of Representatives!"

"Sure you are, baby," he replied as he pursued her. "Your house or my house . . . I don't care what house."

Multimillionaire W. Clement Stone has been paying $300 an hour, or more than $50,000 for the week, to tie up a rented 106-foot yacht, The Helene, on the Detroit River behind the Joe Louis Arena.

He has a full-time staff on board including a hostess, to serve hungry or thirsty VIPs who want a respite from the convention.

But the 78-year-old Stone, who likes publicity, has been having a hard time with some of his guests, who refuse to set foot on board for so much as a sip of chilled champagne unless he promises no press.

"Donny and Marie stamped their feet like children and wouldn't even come aboard the other night when they found out there was a television camera crew here," he lamented on Tuesday night.