Question: Who could get a Kennedy, a Rockefeller, a Harriman and a Mondale to throw them a high-powered Washington fund-raiser?

Mark Dayton could.

Mark Dayton?

Last night, Mark Dayton, a little-known Democratic candidate for the U.S. Senate in Minnesota, had a lot of very well-known Democrats singing his praises and pushing his candidacy at a $100-a-person fund-raiser held at the Folger Library.

Question: So why would the governor of West Virginia support a 34-year-old candidate in Minnesota, before the primary -- a candidate who has never even held public office before?

"Well, for three reasons," said Gov. Jay Rockefeller, who attracted more attention last night than the candidate and Teddy Kennedy put together. "For one, I like Democrats, second, I believe in him, and third, he's married to my sister."

Question: Why does the heir to the Dayton-Hudson department store fortune, who happens to be married to a Rockefeller, need to have $100-a-person fund-raiser in Washington?

"I decided not to accept any public-interest money at all, and although I plan on using some of my own money, I want to raise a lot of small amounts from supporters," said Dayton, big brown eyes piercing as he talks very slowly. "I only want to have to answer to the people."

Dayton worked as a legislative aide to Walter Mondale (who was out of town last night), when Mondale was a Democratic senator from Minnesota. Most recently Dayton was Minnesota commissioner of economic development.

He's handsome in a preppy sort of way, smiles a lot, has many important friends and he is very grateful. Last night's reception raised $20,000.

"Thanks so much for coming," he said to Kennedy, squeezing the senator's hand.

"This is a nice group you have here," said Kennedy. "You're off and going. You're doing well so far."

"I'm the only candidate so far," said the only candidate.

Political wisdom has it that Dayton will at least be running against incumbent Sen. David Durenberger (R-Minn.). He's in the seat occupied by the late Hubert Humphrey. It is expected to be a tough race, costing the candidates at least $2 million each. There has been some speculation that former senator Eugene McCarthy will run as a Democrat or an Independent.

"I generally don't support candidate before the Democratic primary, but I have committed to Mark," said Sen. Alan Cranston (D-Calif.), who flew to Minnesota over the weekend for another fund-raiser. "I believe he will be the nominee and sometimes we need to do things we wouldn't ordinarily do to support the strongest candidate we can."

After the Folger reception, most of the 200 guests went out in the Washington rain and headed home. Except for 40 or so. They headed to an intimate, fund-raising dinner for Dayton at Averell and Pamela Harriman's Georgetown home.

"Potential big contributers," whispered a Dayton staff aide, rolling her all-knowing eyes, as only a political fund-raising staff aide can do.