To hear them talk, they were a roomful of winners, not especially surprising in a Kennedy living room.

But instead of politics it was tennis that spurred the competitive egos at Hickory Hill last night. Ethel Kennedy's dinner guests, while not appearing to be packing carbohydrates, were nonetheless preparing for the Washington branch of the Robert F. Kennedy Memorial Pro-Celebrity Tennis Tournament scheduled at the Regency and McLean tennis clubs for today.

This is not the star-studded version that annually occupies the famous stadium in Forest Hills, N.Y. This is what tournament organizers call a "satellite" tournament, with Washington-style celebrities teaming up with corporate executives whose firms kicked in $2,500 each for the privilege of playing. The money raised goes to fund RFK charity programs in the Washington area.

Ethel's brother-in-law Teddy arrived with a woman he introduced as "my beautiful sister, Jean (Kennedy Smith)." Sister Eunice had already arrived, with husband Sargent Shriver trialing along later. Numerous younger Kennedys and Kennedy dogs - several resembling their late relative Freckles - were underfoot.

The Massachusetts Democrat, whose high visibility this week has included a sharp exchange with President Carter over energy policy, said he thinks the president "understands there are going to be differences on policy." But he added "he'll find by the end of this Congress there'll be few stronger supporters."

Eunice, whom Kennedy had laughingly whistled over to assist him, completed his sentence as he went off to use the telephone. "Than myself," she said he meant to add.

The crowd - glittering by Washington standards - included half a dozen member of Congress, Attorney General Griffin Bell, Baseball Commissioner Bowie Kuhn, and corporate executives from ITT, GEICO, Marriott, TWA, and other donor companies.

Another guest was former Nixon appointments secretary Stephen Bull, now working for Phillip Morris.He joked with colleague Thomas Ahrensfeld, who is to be partnered with Sen. Thad Cochran (R-Miss.) in the tournament.

"We expect to win tomorrow." said Ahrensfeld.

"Well, you know with my background, I fixed it," answered Bull. CAPTION: Picture, Sen. Edward Kennedy, Art Buchwald and Ethel Kennedy; by Joe Heiberger