Nevada's only congressman, Rep. James D. Santini, calls it "a friendly Nevada tradition." Gambling, that is.

He and his wife, Ann set out to prove just now how friendly the other night by turning their split-level home in suburban Potomac into what the WCTU once might have called a "model" den of iniquity.

Throughout the various levels, including the screened porch, were a dozen tables of blackjack, poker, craps and roulette. Downstairs beneath posters provided by Harold's Club was a bar to be bellied up to, upstairs a hot rotisserie (Coney Island variety) that kept running out of hot dogs.

The Santinis who, themselves, never gamble ("Well, that's not quite true, I've tried it once or twice," said the Reno-born congressman) handed out $500 bills at the door as if they were going out of style.

"I had an awful time finding it," confided Ann Santini, meaning the several million dollars in "Hunky-Dory Smackers" she finally tracked down at Garrison's toy store.

Besides the $500 grubstake for each guest, another greeting a the door was the Santinis' plumber, an admittedly unscheduled last-minute, if very welcome, addition. Ten minutes before the party began the kitchen sink stopped up. Getting it unstopped helped make everybody feel right at home. Right away.

The Santinis invited 250 people, and 200 of them came from The Hill, agencies, the media, the neighborhood, special-interest groups and even Nevada, which strictly speaking of course, is a special-interest group all by itself.

Nevada's senior senator, Howard W. Cannon, and his wife and sister-in-law paid a visit though not to try their luck, make-believe or otherwise. "Sure, I'm a supporter." Cannon said of the state's largest industry, which lures upwards of 22 million tourists to its gaming tables every year, "but I learned a long time ago that if you can't afford to lose, you can't afford to play."

Sam Boyd, who owns two Las Vegas hotel-casinos and one casino-club, each netting $1 million annually, he said, drove up in a rented back limousine which had met him at the airport because otherwise "I'd never have found my way here."

Like Cannon and Santini, Boyd never gambles either, except when he travels. Then, in places like France or Tahoe, he may drop $100 on roulette, depending upon his mood. Real money, of course, none of that Hunky-Dory Smackers stuff.

For reasons quite different from those of the Santinis' other guests, Boyd felt right at home, too. Almost immediately after he arrived, he was shown to a fledgling crap table and thrust into the role of croupier, a task he worked his way up from - and out of - years ago, he said.

"I'm the oldest active casino owner in Las Vegas," he said, a hint of pride in his voice that was not without some justification. Contrary to what some people think, El Rancho Vegas, which opened in 1941, was the granddaddy of clubs along the now-famous Strip - not the Flamingo, which Benjamin Siegel (the late "Bugsy" of Murder, Inc., and other connections) opened in 1946. Boyd knows because he was there for both openings.

But Siegel, like organized crime, is hardly a Nevadan's favorite topic of conversation. Although Nevada authorities claim they got rid of the mob a decade ago, recently there has been increasing evidence that the Mafia owns a quarter of one of Las Vegas casino and is known to oversee some prostitution, narcotic sales and loan operations along the Strip. Despite those relevations, Sam Boyd said categorically, "There is Mafia connection - I think Las Vegas has a nice name.

"There are a lot of misconceptions about Nevada," said Cal Sunderland of Winnemucca, publisher of the Humboldt Sun.

"We got a lot of publicity-traded companies and there's no way of telling who's buying in," said Cannon, "but gaming is well controlled in Nevada. A person has to be absolutely clean to get a license. Bugsy Siegel could never get a license today."

Santini, a two-term Democrat once named Nevada's outstanding young man of the year and another time nominated as outstanding public defender in the country, laughed out loud at the notion that there was any motive other than good will behind his Friday-night party.

"I wouldn't do that to my friends," said Santini, who wore Western-cut blue jeans with matching vest.

The party didn't stop his friends, though from making good use of the occasion, that grand old tradition known in Washington as lobbying.Large blue buttons asking "13% Enuf?"-superimposed across a map of Nevada-had been sent by somebody out west. Ann Santini added them to the prizs, at $500 each (in play money) a bargain among what Santini called "the rottenest staff" he could find in his Saturday morning canvases of yard and garage sales.

For the uninitiated, 13% meant the propotion of Nevada land in private domain, the other 87 per cent being under federal control or ownership. It is an issue gaining some momentum with the recent creation by the state legislature of a select committee to do something about changing the ratio.

"Some parcels might be offered up immediately," said Tim Monroe, a Nevadan with the Interior Department's Bureau of Land Mamagement which will help identify the Nevada lands for the committee. "Others may take 10 to 15 years, at least."

Just the other day, Santini introduced HR 4747, a bill calling for the return at "fair market value" of 2,500 acres by the federal government to Mineral County, Nev. With only 1 per cent of the county's land in private ownership, Mineral County's lack of a tax base is the classic example of why gambling was legalized in the '30s.

No party, though, is without its skeptics, and the Santinis' was no exception. "If the federal government ever did give the other 87 per cent back to Nevada, those people wouldn't know what to do with it," predicted a committee staffer from the Hill . . . just as some guests apparently never quite got the hang of parlaying their $500 into the make-believe good life.

"I don't know how anybody could make any money the way they're doing it," said Sam Boyd, the former "box man." "floorman" and "pit boss," rolling down his custom-made shirt sleeves and stealing off into the night.