The most competitive heavyweight championship fight in 6 1/2 years is expected to take place in Las Vegas Friday night. Across the nation, several million people will be watching on pay television, cable television and closed-circuit big screens. Or will they?

Advance-sale tickets for the Larry Holmes-Gerry Cooney title bout are setting no records in key markets, including New York and even Boston, where Cooney's Irish mug was supposed to spark a firestorm of Celtic chauvinism. The closed-circuit contractor for Washington and Baltimore says advance sales "are moving, but not as fast as some other fights."

The sluggishness is a shocker to many who viewed this as at long last a heavyweight showdown in which the outcome was in doubt. "People get excited over the chance of a change," said Lou Falcigno, whose Momentum Enterprises is handling marketing and promotion of the fight.

In Holmes, the champion who has suffered from a pathetic lack of competition during his four-year reign, and Cooney, the massive but untried slugger from the suburbs of New York, the likelihood of an unexpected shakeup seems better than in any title fight since Muhammad Ali and Joe Frazier answered the bell in Manila in 1975.

So they scheduled some live preliminary fights at Madison Square Garden, where Cooney pounded Ken Norton into oblivion in one round a year ago, printed up tickets for the combined live/closed circuit show at $30 to $60 and waited for the lines to form.

They're still waiting. "It's been slow," said Tommy Kenville, the Garden boxing publicist. "There's a lot of resistance. From what I hear it's moving slowly everywhere."

The antipathy puzzles Kenville. "A white, unbeaten heavyweight is supposed to be what everyone was looking for. But Cooney didn't draw that well here against Norton (10,000 paid) and he didn't sell out against Ron Lyle in his own backyard, the Nassau Coliseum. When he was coming up he couldn't sell out our Felt Forum with 4,000 seats.

"I can't figure out the reason. We're hoping it'll be a last-minute thing. Maybe we'll sell out. Who knows?"

Almost everyone agrees on a few basic reasons for the slow going.

Postponement: The bout was scheduled for March 15 but Cooney injured a shoulder. Postponements always play havoc with the appeal of a fight; in this case the whammy doubled when the delay knocked out the attraction of an Irish challenge two days before St. Patrick's Day.

"I see this fight as less a black and white thing than an appeal to the ethnic pride of the Irish," said Bert Sugar, publisher of Ring Magazine. "Boxing used to be the special province of Irish-Americans and here you had a chance for an Irish champion for the first time since James J. Braddock," who lost the title to Joe Louis in 1937.

Also, Cooney's injury left some fans with lingering doubts about his health and concern that the fight might be postponed again.

Hard Times: "Things are not that fantastic" throughout the amusement industry, according to Irving Squires of Shady Grove Music Fair Inc., which is handling closed-circuit outlets in Washington and Baltimore. Squires said inflation and unemployment cut deeply in his business. He's counting on the fact the fight is on a Friday--payday for many fight fans--to pump up last-minute sales.

Counseled Jim Merila, a consultant to King-Tiffany Productions which is selling the fight to local promoters: "Fight fans are traditional last-minute buyers. They'll buy next week or walk up the night of the fight."

But it isn't always a last-minute binge. Squires remembers Sugar Ray Leonard fights where three-fourths of the tickets were gone a week before the bout. By contrast, he said, only 20 to 25 percent had been sold a week before Holmes-Cooney.

Who are these guys?: Cooney has fought exactly six rounds in the last 2 1/2 years and some of those weren't even complete rounds.

He knocked out Jimmy Young in four rounds and Ron Lyle in one in 1980, and obliterated Norton in 54 seconds in 1981. That's it. None of these opponents was in his prime. "I'm the only guy over 40 he hasn't fought," sneered magazine man Sugar.

Holmes has been busy, defending 11 times the title he won from Norton in 1978. Unfortunately, for all his activity his workmanlike achievements have failed to capture the fancy of fight fans. Sugar, again mincing no words, said: "Larry Holmes can't draw flies."

There are reasons. Holmes is a tactician; his best punch is a jab that wears opponents down, not a knockout haymaker. His private life in Easton, Pa., is abnormally normal; his opponents over the last two years (Reynaldo Snipes, Leon Spinks, Trevor Berbick, a washed-up Ali, Scott LeDoux) have been uninspiring, through no fault of his.

It is interesting that as undefeated champion, Holmes, 32, has yet to get a national advertising deal. Outside boxing he is essentially unknown. At 25, the dark-whiskered Cooney is already one up. He and his mother began appearing on national television recently in an ad for electric shavers.

Despite the slow sales start, no one expects this fight to be a financial bomb. The boxers are guaranteed a purse of $3 to $5 million apiece, according to reliable sources, although figures as high as $10 million apiece have been cited.

Caesars Palace paid $5 million to stage the fight in a parking lot surrounded by 32,000 seats priced from $100 to $600. All but a number of $200 seats and a few $500 seats were sold by last week, the hotel reported.

Merila said closed-circuit rights brought promoter Don King $7 million to $8 million, pay TV rights yielded $5 million and cable another $1 million. Adding in foreign TV rights, more than $20 million is already in the bag.

The potential total gate, should closed-circuit promoters sell all 2 million of their seats, is close to $50 million, said Falcigno, who remains optimistic about a big payday for everyone.

Said Falcigno: "Cooney demolished Norton on Home Box Office and national television. A few months later (six, to be exact) Reynaldo Snipes knocks down Holmes on national TV. People start saying, 'Hey, wait a minute. We know Cooney can hit hard. If Snipes can knock Holmes down, what will Cooney do to him?' "

But if people are saying such things they're doing so very quietly. Meantime, the hour approaches.

The cable/pay TV/closed-circuit grid may be a victim of its own greedy nature. Making each viewer pay is clearly the best way to make a pile of money out of an event. But it also short-circuits the best system for hyping that event--national media.

If network TV were broadcasting the bout it would be getting star billing on promos every half-hour on all the network stations, nationwide. The network would buy additional national advertising, interview participants on its news shows, invent animosities, shower the press with advance information and offers of interviews and generally hype the show shamelessly.

Instead, promotion is essentially being left to local entrepreneurs, whose stake is strictly in filling local theaters or in selling pay or cable TV services to local buyers. "Frankly," Falcigno said, "all we're doing is letting people know where they can buy tickets."

So far, according to those doing the selling, that's not enough.