For nearly a decade, Eddy Merckx, who grew up here and rode off to become the greatest professional bicycle racer, has dominated a sport that has for a half-century been more popular than any other in Europe except soccer.

European poll-takers tend to be not as well organized as their counterparts in the United States, but some idea of pro bicycle racing's popularity can be gleaned from a poll taken recently by the Italian newspaper Stadio. It indicates that 21 million people in Italy follow the sport avidly about half the population.

In France, the gendarmerie estimated last year that 15 million people, one-third of the population, walked off their jobs if necessary and lined the 2,500-mile course to watch the three-week Tour de France in July. In the other five countries that have traditionally supported the sport - Holland, Belgium, West Germany, Spain and Switzerland - the numbers are similar.

So hundreds of millions of Europeans are turning over in their minds the two important questions of this year's pro racing season, which began Feb. 3 (with the six-day I'Esoile de Bessegues, on the Cote d'Aznure) and runs nonstop until the end of October. In that time span, many riders will race daily in any of the 150-odd tests scheduled.

According to the professional cyclists association, about 450 men make their living on the seat of a bicycle. The first question is, "Will Eddy Merckx win the Tour de France?" The other: "Why does he bother to try?"

Now 33 years old, Merckx has done more in his 12 years as a professional racer than any other man in the history of the sport - won more races, set more records, collected more money and probably displayed more courage. What more does he want? fans ask. What more can he?

Last May, Merckx announced his retirement, citing his declining fortunes since finishing second to France's Bernard Thevenet in the 1975 Tour de France, his only Tour defeat, alas, in his last five attempts. In June he reversed his decision and promised he would make his final attempt to break the five-victory Tour record, which he shares with the retired French great, Jacques Anquetil, in the 1977 Tour in July.

In a remarkable effort, which did little to tarnish his "Mr. Courage" image, the found off no only 99 other riders but a severe stomach infection to finish sixth overall. Still, it was his poorest showing in the event, and his faithful following begged him to keep his promise to retire, emphasizing that both the 1976 and 1977 seasons had been, by comparsion with his prime years, disastrous. He did not. Near the end of the 1977 Tour, he announced he would be back in 1978.

Since winning his first Tounr in 1969, Merckx has become the star attraction of the racing circuit. His take for merely showing up at a race runs to $2,500, and his annual income from racing and related activities has been put at $500,000.

What he calls, therefore, "the problem of money" he also calls "secondary." The son of a Brussels grocer, he now lives comfortably with his wife and two children in Kraainem, a wealthy suburb of the Belgian capital. His life style is modest. The principal reason he continues to compete, he says, is pride.

"I've thought about it a long time, and I've decided that I didn't lose the Tour de France last year on merit. I was ill. So I want to prove that I haven't - as some people have said - ridden one Tour too many, and that I'm still capable of winning."

If he were, on the other hand, to retire now and rest on his laurels, he would rest well. Since turning pro in 1965 (he has won every major race at least once - from one-day "classics" like the Milan San Remo (seven times) to weeks - long "Tour" like the Tour d'Italic (five times) - racking up some 75 victories all told.

His Tour de France feats are astonishing. Besides winning it five times, he has twice equalled the record for number of stage wins (eight), winning 34 stages in all, a record notlilikely to be broken in the foreseeable future. No riders competing today even come close.

Second to Merckx, with 25 stage wins is the French racer Andre Ledqucq, and he retired in the early 1930s. Merckx has won the Tour de France points competition (the ranking based on stage finishes) three times, a record he shares with Holland's Jan Janssen, and in 1969 he did something no racer is likely to do again - won the points and the mountain-climbing prizes in the same Tour. Merckx has alos held the workd one-hour record (49.431 kilometers) since 1972. He has won the World Road Championships no less than three times.

"What I have to avoid above all," he said in late 1975, "is hitting a decline - the begining of mediocrity."

To many fans, that beginning came, in fact, in 1976. In the Tour d'Italie (June) he sustained a saddle boil that forced him to withdraw from the Tour de France the following month, and the rest of the year followed suit . . . downhill. Last year, after launching the season hopefully by winning the weeklong Tour Mediterranean in February he peroormed poorly during the rest of season, winning no major races.

"If Eddy has decided to compete again in the Tour de France," says the event's director, Jacques Goddet, "it is because he is sure he can win." Others remain skeptical, including Jacques Anquetil.

"If I were in his position," Anguetil said, perhaps protective of his share of the Tour record, "I would retire. He wants revenge after his loss last year. But he doesn't need it. People will still admire what he's done in the past."

That appears far from Merckx's mind. More confident than ever, he announced that negotiations with a British firm to sponsor his team had broken down (over money) but that he had found, in late January, a Dutch sponsor. His new team, to include 1976 Tour de France winner Lucien van Impe and his longtime chief "lieutenant" Josef Bruyere, will enter all the spring "classics," the Tour de Suisse and, of course, the Tour de France.

What does Eddy Merckx want? "He wants," says his wife Claudine, "to leave racing without regrets."

Even if he had finished the 1977 Tour de France two or three minutes behind the winner instead of the 12 minutes 38 seconds that he did, she says, he would have retired.

Her husband puts it this way: "I don't want to leave by the back door. I don't want to be remembered by the public as simply a rider in the bunch but as a leader."

That he would ever be remembered as a follower is doubtful. But this season will be Merckx's moment of truth - for him. Will he be forced to leave racing "by the back door"? He has, after all, announced his "definite" retirement for the end of the 1978 season.

Or will he rise to the ocasion - as he has done in the past - and win his great reward . . . to leave by the front?