Walter Mondale, not surprisingly, was the main attraction last night at a grand black-tie dinner honoring the memory of his Minnesota mentor, Hubert Humphrey.

"This is just out of this world," said Muriel Humphrey Brown, her voice wavering in the marbled elegance of the Capitol's Statuary Hall. "I sure do get all choked up at these times."

Two hundred Minnesota supporters, as well as some old-line backbones of the Democratic Party, congregated on the eve of the posthumous presentation of the Congressional Gold Medal to Humphrey. President Reagan is scheduled to present the award to Muriel Humphrey Brown in the Rose Garden this morning.

Under a gleaming chandelier the size of a Christmas tree, the likes of Edmund Muskie, Robert Strauss and Virginia Gov. Charles Robb ate veal, drank champagne and toasted Humphrey's legacy.

They also boogied quite seriously to a loud and large band.

But first, this:

George McGovern's advice to Walter Mondale: "He's got to go get these debates with Reagan. Two or three televised debates could turn this thing around. Otherwise it's going to be a very tough campaign. It all depends on that."

Chuck Robb's advice to Walter Mondale: "He suffers by comparison to the president in the area of likability. He has got to find a way to present the real Walter Mondale. He doesn't come across with the same kind of warmth that Reagan does."

Ann Lander's advice to Walter Mondale: "Hurry up. It's getting pretty late in the game."

Walter Mondale's advice to Walter Mondale: "Be myself. Otherwise you forget."

When reminded that being himself was the one thing his supporters maintain he can't seem to get the hang of, the Democratic presidential nominee let out a hearty guffaw. "It's coming!" he said. "It's coming!"

Mondale gave the main toast in honor of Humphrey, repeating his 1978 eulogy to the former senator and vice president.

"He taught us how to live," he said, "and in the end, as I said on the day we said good-bye to him, he taught us how to die . . .

"Hubert taught a generation to be compassionate," Mondale said. "Hubert taught a generation of political leaders to respect American people . . . Hubert taught us to be strong . . . Let us not toast just our memory of him, but also what he told us that lives in us today."

The dinner was given by Muriel Humphrey Brown and Dwayne Andreas, a Minnesota millionaire industrialist and chairman of Archer Daniels Midland Co. Halfway through the meal, the Gene Donati Orchestra wended its way through the tables playing show tunes.

"Gosh, this is just like Las Vegas," observed one Democrat. "I've never seen this done anywhere other than Las Vegas."

Hearing that said in earshot of a reporter, one of the planners of the event cringed.

It was one of only seven dinners ever allowed to be held in Statuary Hall. The first was Lafayette's return from the Revolutionary War, and the last was a bicentennial affair for Queen Elizabeth II.

The main prerequiste for securing permission to use the hall is that the event be bipartisan. Sen. Robert Dole (R-Kan.) preceded Mondale in toasting Humphrey: "Hubert Humphrey was certainly one of the biggest men I have ever known."

In the end, Muriel Humphrey Brown nervously took the microphone. "This night is a great privilege," she said. "I've had many great privileges in my life. And one of them was being married to Hubert."