Chukkers and champagne--enjoy them both this Sunday at the Potomac Polo Club, where the polo ponies will be galloping up and down the field while galloping chefs and gourmets vie for the title of best tailgater on the sidelines. Elegant, bizarre, weird or wonderful, the tailgate parties will be judged in four categories -- style, food, theme and originality. First prize for the best overall party is a silver bowl followed by polo lessons, champagne, dinners for two and a night at an inn.

The contest at the Potomac Polo Club is open to all. Admission to the grounds is $3 per person, and anyone picnicking at the site between the 4 o'clock start of the polo game and half-time will automatically be entered and judged in the tailgate contest.

Among those who'll participate are members of several car clubs, including the Rolls-Royce Club, sponsor of the polo match between Gone Away Farms and the Potomac club, and the Bentley, Jaguar, Austin-Healey, MG, Triumph, BMW and Ferrari clubs. There'll also be some antique cars, including a '29 Packard with jumpseats, a '28 Lincoln and a 1920 "woody" station wagon owned by Claude and Patricia Owen of Potomac, who last year presented the winning "Roaring Twenties" tailgate party. Winners will be announced at the end of the match.

"That day is when you really do everything up -- you know: your big English picnic basket, silver ice bucket, lovely linens and lots of good food," said Iris Arnold. She and her husband, William, a retired Navy captain, "don't really care whether we win or not, but it's fun to make it such a nice party," she added.

Nearby at the Potomac-vs-Middleburg match, a group of four were also discussing their plans for Tailgate Sunday. Between sips of a mimosa and nibbles of p.at,e, James Lee Barnhart II of Bethesda said, "The only problem we're having is deciding whether we'll do it in black tie or as Jamaican Rastas. But anyway, it should be fun."

All this sidelines activity is not to imply that members and friends of the Potomac Polo Club aren't serious about their polo. They are. "Most of us own and ride horses and enjoy coming out and spending a relaxed Sunday afternoon," said Steve Spector of Gaithersburg, adding: "If you love horses like we do, then this is great entertainment."

Polo is like four-man hockey played on horseback. The charm of the sport isn't in the strategy or rules, which are kept to a minimum, but in the athletic coordination between horse and rider. A good polo player controls his horse through a complicated combination of skills -- easing forward or pulling back on the reins, shifting body weight and pressuring the horse's sides with calf and heels, often with the help of mild spurs -- all designed to get the animal to respond quickly to changes in direction and speed. The rider also has to wield a four-foot mallet and often a whip.

The object of the game is to hit the ball through one of a pair of goal posts on a field that's 350 yards long and 150 yards wide. As you sip your fluted crystal glass of champagne or your plastic cup of Sprite, you'll be surprised how often the two teams thunder past, hooves flying as the riders lean forward with their sticks held high like battle pennants.

Polite applause often greets an outstanding effort, but please, ladies and gentlemen, no raucous cheering. Booing is taboo. The key to being a proper polo spectator is to maintain a casual detachment from the roaring, sweating spectacle. During halftime, some spectators come out onto the playing field to nonchalantly replace the divots kicked up by the quickly cutting horses.

For many, watching is the only way to participate. To get into the game, you not only need to be fearless and talented in the saddle, you also need to be pretty well off. For a normal 11/2-hour match of six chukkers, or periods, each of the team's four players needs a fresh mount each chukker. That means the team must have access to at least 24 properly trained and cared-for ponies and a support staff of no less than four grooms.

This pricey angle probably explains why polo is often a family endeavor. There just aren't a lot of individuals who can rustle up the resources.

This Sunday, for example, Gone Away Farms will be represented by the Muldoon family in the match against the Potomac club for the Rolls Royce trophy. A key member will be Joseph Muldoon Jr., 51, an area attorney and polo promoter who owns the grounds on Hughes Road just off River Road, up in the rolling green hills of Poolesville, where the club holds its matches. Riding with him will be two sons and perhaps a daughter.

While watching the Muldoons take on the best Potomac has to offer, spectators -- especially the tailgaters -- are warned to stay alert and well back from the field: Horses do occasionally gallop over the sideline boards in pursuit of the very hard plastic ball.

And horses' hooves in the hors d'oeuvres are strictly outre. PICNIC WITH THE PONIES Potomac Polo Club games are held every Sunday through October 2 at 4 (gates open at 3) at the Hughes Road grounds directly off River Road near Poolesville -- look for the large signs. Admission is $3 per person, children under 12 free. Free to all members and season-pass holders, including three guests. Fieldside parking the day of the game is on a space-available basis at $5 per car. First come, first served. Refreshments available on the grounds. For game schedule when rain threatens, call 703/849-8222. For information about lessons, call 972-7241 or 703/849-8222. TOWN AND COUNTRY POLO-- The National Capital Polo Club holds free matches most Sundays on the Lincoln Mall Polo Grounds, between Independence Avenue and Ohio Drive, west of the Tidal Basin, from Memorial Day through November, weather permitting. The next scheduled Sunday game is on September 11 at 4. There are 400 chairs set up by the announcer's stand, available on a first-come, first-served basis, and parking's available on Ohio Drive. Call 426-6700 or 426-6841 for information. Out of town, the nearest polo clubs are in Middleburg and Charlottesville. Both clubs play regularly at Potomac Polo Club. POLO PRIMER -- Marcus Bignoli, an Argentine professional polo player, conducts a polo school on Saturdays at noon at the Lincoln Mall Polo Grounds, complete with horses and practice chukkers. For information and rates, call 425-5757. graphics/illustration: ponies By Robert Soule