Well, it is June, but the invitations went out in May, and they were married in December. Which, you might say makes them the U.S. Senate's most famous May-December couple for at least two reasons. The other, of course, is that Strom Thurmond, the South Carolina folk hero, is 77 and his wife, Nancy the former beauty queen, is 33.

Last night it wasn't the durability of their own romance they were celebrating, however, but that of Thurmond's in-laws, Julie and Paul Moore of Aiken, S.C. who were married 43 years ago in Washington. To mark the occasion, the Thurmonds invited in several hundred of their closest political, diplomatic, military and industrial friends.

"Anybody who's anybody is mostly here, aren't they?" noted Thurmond with undisguised pleasure.

And they mostly were.

For one thing, there were the Thurmonds' four young children, all scrubbed within an inch of their lives and looking like fugitives from "Little House on the Praire."

But beyond that the throng welcomed into the Senate Caucus Room by the Thurmonds and the Moores was a cross-section of Washington clout that lately has been playing the nightly news on such volatile subjects as the budget, defense spending and military preparedness.

The commanding figure of Thurmond's Democratic colleague from South Carolina, Sen. Ernest F. Hollings, towered over a group of foreign ambassadors lionizing him in a manner befitting the chairman of the powerful Senate Budget Committee.

"Heck no," Hollings said to inquiries about whether he had heard from the White House since he called Jimmy Carter's vacillations over the budget "outrageous and deplorable conduct" and "the height of hypocrisy."

Not far away was Gen. David C. Jones, chairman of the Joint Chiefs of Staff, who told a House Armed Services subcommittee last week that the United States was playing "catch-up ball" militarily with the Soviet Union. uDespite what anybody might have thought from his previous testimony on defense budget needs, Jones said he has been "strong for defense all along."

And while one South Carolinian in the crowd sniped that Jones' reappointment by Jimmy Carter for another two-year term provided him some security he hadn't had earlier -- "he wouldn't open his mouth before" -- Jones maintained that he stood on "what I said before the committee in my constitutional role [as adviser]." Another of the joint chiefs at the party, Gen. E.C. Meyer, Army chief of staff, had also taken issue publicly with their commander-in-chief.

Then there was Adm. Hyman Rickover, professing to be "just a servant -- I don't enjoy politics." In fact, Rickover, sometimes called the "father of the atomic-powered submarine," dislikes politics so much that, he said, he never votes.

"I stay out of politics," he said matter-of-factly without even a hint of apology in a room famous for its politicians. But he did have some kind words for Carter, who, he said, at least tries.

"Even Jesus Christ didn't please everyone," observed the 80-year-old Rickover, "and still doesnt."

But while many names on the guest list might have been staight out of a Senate roll call, others, like former senators Hugh Scott, Jack Miller and John Sherman Cooper, were straight out of the past. Including Charles Colson, a born-again Christian and fellow Baptist from the Columbia Baptist Church of Falls Church that the Thurmonds attend.

Yes, certainly, Colson had read Spiro Agnew's new book, just as he's read most of the books on that period in time. Agnew's was especially intertaining, he said.

"I was his lawyer for much of that time, in on most of the discussions. If he was worried about his life he didn't tell me, and since I was his lawyer you'd think he would have, wouldn't you?"

Back in the receiving line the Thurmonds and the Moores were hard at it. Julie Nylund and Paul Moore had been high school sweethearts in Butte, Mont., a romance they continued at the University of Washington. When Paul graduated and came east to work for DuPont, Julie could hardly wait to join him. She did in 1937 and the reason they chose Washington for their nuptials was because there was no three-day waiting period for a marriage license.

Paul, retiring this year after 43 years with DuPont, said he never did ask Julie's father for her hand. "I wouldn't have dared do that," he said."I'd have been too scared."

Of his son-in-law, Strom, 12 years his senior, who after several hair transplants had what some might call a head start on him, Moore had only praise: "Very thoughful, considerate and a gentleman at all times."

As for Thurmond, whose arm lay affectionately around Nancy Thurmond's bare shoulders throughout much of their receiving-line duty, he was looking forward to even more hair, apparently.

"I may be out to see you for a couple more transplants," he told Dr. Ronald R. Cameron of Rockville, who helped make him the man he is today.