"When Ethel asks you out," said the lawyer, smiling, "as an old friend, you come."

In fact, when Ethel Kennedy, widow of Sen. Robert Kennedy, asks anyone to anything they usually come.

So last night, a few hundred persons walked up the graceful curve of driveway in front her McLean house, known as Hickory Hill, past the little sign on the front lawn saying "Trespassers Will be Eaten," into the cool airy house.

The occasion was a $100-a-person fund-raiser for Sen. John Culver (DIowa). And each of the guests-his staff, his friends, about 100 Iowans here from out of town, and a variety Washington lawyers and professionals-was personally greeted by Culver, his wife Ann, and Ethel Kennedy in a reception line in the hallway.

"I imagine she has to be discreet about who she gives fund-raisers for," said one guest. "This is one of the prize places to have a fund-raiser."

The Culvers and the Kennedys have a long and friendly history, dating back to the years when Sen. Ted Kennedy and Culver were football teammates at Harvard.

"What the Culvers want, the Culvers get," said Ethel Kennedy breathlessly with a big smile. "I only wish this was for his presidency."

Ann Culver turned to her in surprise.

"Oh, yes," said Mrs. Kennedy.

But would she prefer him instead of Ted Kennedy, she was asked.

"Well . . ." she said, taking a deep breath and pausing, "I don't talk about Teddy."

Culver, who went from 10 years in the House to a Senate seat in 1974, is running for reelection at a time when conservative Republicans are making special efforts to win his seat and those of a few other targeted liberal senators.

Liberal Democrate and former senator Dick Clark of Iowa fell by the wayside in the last election. (Many people from "the Clark list"-supporters-were invited to last night's fund-raiser.)

Ted Kennedy arrived midway through the party, moving easily through the house, greeting everyone with "very nice to see you." He smiled, paused to talk, and laughed heartily in bits of conversation. Instamatics flashed in his trail.

"I know Ethel wanted to do something for John," said Kennedy. "She has great respect for him. As for the race, he's taking it very seriously, leaving nothing to chance-just as he has always done. If the vote were up to the Senate he'd win over-whelmingly."

Some of the Iowans there had also come up for a regional UAW meeting, and one group had had lunch with Culver and had gotten a chance to meet Vice President Mondale.

"If it's John Culver, we're here," said Dixie Allen of Montrose, Iowa, who works at the Schaeffer Pen Plant there.

"The companies don't support him, but the unions do," added Don Pribble, who works for J. I. Case (which) makes tractors) in Davenport, Iowa. "He cares about the blue-collar worker."

They are prepared to fight for his re-election, they said soberly.

But meanwhile, they had fun roaming Hickory Hill, with Kennedy children and a variety of dogs (one guest said she counted eight) underfoot.

"Can I have a picture with you, Ethel?" asked Pribble.

"You bet," she said. He quickly assembled Ethel and Sen. Culver and Dixie Allen, arms all linked, readied his flashcube camera, and snapped. CAPTION: Picture, Ann Culver, left, Ethel Kennedy and Sen. John Culver; by Ellsworth Davis