Sooner or later it was bound to happen -- turning the White House into a theme park. And last night, using a bright red barn, the prow of a New England schooner and a picture of a gigantic paddleboat for backdrops, they not only converted the South Lawn into a Disneyland-on-the-Potomac, but they even called it "A Salute to Congress."

We hope to see you many more years here on the South Lawn of the White House," President Carter told the crowd of more than 1,000, which included members of the House, Senate and Cabinet and their spouses. And they didn't have to worry about a lot of political speeches, he assured them.

"We made everybody check their political speeches at the gate, so there is not going to be anything tonight but good food, good dancing, good friendship," Carter said.

Politicking hadn't been checked, of course, and that was carried on in the fine old tradition of presidents before him with handshakes for the men, kisses for the ladies and pauses for photographs for almost anyone who wanted them.

Quite a few wanted them, especially the out-of-town chefs who had set up buffet tables loaded with such regional specialties as Maine lobsters and clams, barbecued spareribs and pork and tacos and chili con queso.

Restauranteur Larry E. Sconyers of Augusta, Ga., said that when the White House approached him two weeks ago about doing something for Congress, "we offered our services" (as well as the food) willingly and withoout charge. Taking the publicity potential into consideration, Sconyers indicated that he expects the deal to pay off in fame if not money.

It took Sconyers' mother, Adeline, however, to confide that "it's costing an arm and a leg."

Unaware, perhaps, that the chefs and their hometown supporters were picking up the tab for much of the evening's food were the objects of everybody's affection, the bipartisan crowd from Capitol Hill. They included the U.S. Senate's newest and probably least-known member, Maine Sen. George Mitchell, still reeling from his first all-night stand (or sleep) at the Capitol.

"I wanted to tell the president to put me back in a judgeship -- it's still open -- because I didn't care too much for the hours on the Hill," joked Mitchell, referring to the filibuster on military registration, which took a break last night by mutual agreement of the warring factions.

Among the more familiar Senate faces were Jacob Javits of New York, Dennis DeConcini of Arizona, John Glenn of Ohio, Lloyd Bentsen of Texas, Don Riegle of Michigan, Strom Thurmond of South Carolina and Herman Talmadge of Georgia, who created a minor stir until he introduced the pretty young woman at his side as his daughter-in-law.

Skirting the crowd and sidestepping photographers was Rep. Richard Kelly (R-Fla.) of Abscam notoriety, who paused in his flight between beef and beer tables to give Agriculture Secretary Bob Bergland a cheery hello.

When Strom Thurmond spotted Jack Kemp, he detoured long enough to assure the New York Republican and everyone else within earshot that Ronald Reagan would do well, indeed, to seriously consider him as a running mate. Kemp, on the other hand, insisted he hadn't heard a peep out of Reagan.

"Honest to God, he hasn't called," said Kemp. "I know nobody believes politicians, but I'm not in on that process. I'm in to radicalize the economy. But it's not going to happen, he's not going to reach into his own campaign; he's going to reach out. My own hopes and aspirations are to have an impact on the policy, the platform and the ideas and he's given me a chance to do so and I'm really happy."

Javits said the big issue, of course, is whether Reagan will get a moderate as running mate or "someone like himself -- though he says he's a moderate. We've got a great chance to win it (the presidency) and to unify the party."

In the Carter camp, reaction among Carter Democrats (which seemed to be the only type present) was overwhelmingly favorable to news that White House chief of staff Hamilton Jordan will shift to full-time direction of the campaign.

"We'll miss him, but it's the best use of the president's assets," said Presidential Counsel Lloyd Cutler.

And Gene Eidenberg, who becomes presidential assistant for intergovernmental affairs when Jack Watson takes over Jordan's job, dubbed it his "last supper" as he headed in the direction of the barbecued pork.

Japanese lanterns illuminated a dance floor in the middle of the lawn, and taking a few twirls before they headed for the mansion were the Carters, she in a long embroidered skirt and embroidered Mexican tunic and he in powder blue slacks and Mexican shirt.

If the Carters didn't get any lobsters, 500 of their guests did, fresh from open-wood fires where layers of seaweed were stacked with corn and clams under the guidance of James and Gayle Lumsden of Maine.

National Security Adviser Zbigniew Brezinski tied a plastic bib around his neck and dug right in, the same way Defense Secretary Harold Brown did just across the table. ABC's Sam Donaldson did a double take when he spotted Brzezinski.

"You must wear two of those bibs, as sloppy as you are," Donaldson scolded.

Brzezinski nodded indifferently and went right on eating.