It would have been another Washington party celebrating another Washington book except that in this case, another Washington politodrama got in the way. This one was called Muskie! (or: Muskie?) and even though nobody got fired, it was still wonderful fun.

The party was for Samuel Shaffer, Newsweek's former chief congressional correspondent, who's written a book called "On and Off the Floor." Last night, the festivities honoring it gave five attending senators the chance to hash out, only hours after the fact, why Sen. Edmund Muskie (d-Mine) would leave a guaranteed two years on the Hill for what could be no more than eight months in an administration one referred to as "a shambles."

The assessment: First, Muskie wants a change after 22 years; and second, he isn't afraid of being secretary of state in a White House that includes Zbigniew Brzezinski, the more hawkish national security affairs adviser who clashed with Cyrus Vance.

"He's got the strength and the character and the will to stand up to Brzezinski better than anbody I know," said Sen. William Proxmire, the Wisconsin Democrat who said he was "stunned" when CBS called him shortly after noon yesterday for a reaction.

"He's a strong man, and I don't think he's going to get pushed around," said Sen. Barry Goldwater (R-Ariz.)

"I first heard it on the squawk box, all in speeches from the floor about Muskie that sounded like eulogies," said Sen. Larry Pressler (R-S.D.), and my first reaction was, "My God, what's happened to him?' And then somebody told me he'd been made secretary of state.

"I was amazed," continued Pressler, who serves on Muskies's budget committee. "It's very rare that a chairman of a Senate committee will take a Cabinet appointment that many would consider a step down. He must feel that he's serving his country. Maybe he can pull the shambles together over there. Carter's lucky to be getting him."

The author, a man who has written about the Hill eight years longer than Muskies's made law on it, had this appraisal of Washington's latest political entertainment: "Muskie wanted something new," he said. "He's jaded after 20 years here. And if Brzezinski were to even think of crossing Muskie, the whole Senate will be down on Brzezinski. Muskie is one of theirs."

As for the book, it's a first-person account of Shaffer's 30 years of trials and tribulations as a Capitol Hill reporter, and now that he has been retired for a while, there seems to be more tribulation than trial in his memories of what he's called "the best beat in town."

Another memory from this man they used to call the 101st senator: "When I sat up there in the gallery during a joint session of Congress, I would look down on 535 men and women and I saw the cities, the ghettos, the suburbs, the rolling plains. . ."

But to make sure things didn't get too gooey in Senate Room 207, which held Rep. John Brademas (D-Ind.), Sen. Alan Cranston (D-Calif.) and maybe 150 others in the course of the evening, Proxmire got up to the lectern and started talking about boring realities, like good books to read.

"If you really want to make a great investment," Proxmire told the group, "spend $10.95. It's called "The Fleecing of America."

He wrote it.