No big deal, insisted the host. This party among the flamingo lilies and hibiscus blossoms at the Botanic Gardens was just your standard social occasion, a chance for 300 close friends to get together over drinks and South Dakota pheasant pate.

Politics? A candidacy? The White House, perhaps? Wrong, wrong, wrong.

But Sen. Larry Pressler (R-S.D.) sure fooled a lot of his guests last night at a party some expected to be the start of his anticipated White House campaign.

"I got very nervous because I thought you were going to announce tonight," said Phyllis Wyeth, a supporter for the handicapped and the wife of James, the artist.

"It could be interpreted that way," Pressler had admitted earlier in the evening. "But I'm going to meet with my committee once or twice more -- and then decide."

The "committee" is the 50-member Larry Pressler for President group that was organized by Washington attorney Paul Arneson in August. Last night, its members were easily spotted by the yellow "Larry Pressler for President" buttons they wore and the Republican politics they spoke. Like this:

"If Reagan and Connally each get about the same percentage of the votes," said Arneson, "and Larry gets 10 percent, then he'll be the winner because he'll have come from behind."

But at least one guest wasn't calculating percentages or puzzling over the will-he-or-won't-he question. That was Deena Clark, the Washington TV show hostess whom Pressler dates regularly.

"He's going to run, no matter what he tells you," Clark said. "I'm NOT the last one to know."

While all this Republican talk was going on inside, a 1930 Model-A Ford painted with "Ted Kennedy for President" was parked outside. When its owner, Lester Townsend, from Hillcrest Heights, Md., was informed that people like Sens. Edward Zorinsky (D-Neb.), Ted Stevens (R-Alaska), S.I. Hayakawa (R-Calif.), David Pryor (D-Ark.), Howard Metzenbaum (D-Ohio), Harry Byrd (I-Va.) and David Durenberger (R-Minn.) were inside milling around with a possible presidential candidate, he responded:

"Yeah, but he's not the people's choice. I'm telling you, Kennedy's going to do it. People think he's god."

Aside from politics, people talked plants. It was a conversation topic hard to avoid because guests often found themselves walking into giant birdnest ferns or other overgrown houseplants. Still, people marveled at the foliage as they meandered from the South Dakota pheasant at the food table ("I swear it came from Frederick, S.D., and we had an awful time getting it," said Pressler) to a breezy room with a crowded dance floor.

For some, though, the trip was a rough one, filled with all sorts of strange growing things not normally encountered at parties.

"Oh, look at that revolting thing there," said Regina Grasselli, a guest who found herself staring eye to eye with a brownish-green sprawling cactus called a Borzicactus Auriespina. "It's disgusting."

For other guests, the party offered a good chance to make a debut appearance on the 1979 fall social season. Lawyer Steve Martindale was there, as were fund-raiser Esther Coopersmith, the Walter Ridders, the Roger Mudds, economist Eliot Janeway; Ambassador Nicholas Henderson of Great Britain and Ambassador Andreas Jacovides from Cyprus.