ATLANTIC CITY, N.J. — America’s gambling industry predicts $10 billion will be bet on the March Madness college basketball tournament — nearly all of it illegally or off-the-books. That’s one of the reasons the American Gaming Association favors the full legalization and regulation of sports betting in the United States. The group found 54 million people — or about a quarter of the U.S. adult population — participated in a sports betting pool last year. The U.S. Supreme Court is weeks away from ruling on New Jersey’s challenge to a law limiting legal sports betting to just four states: Delaware, Montana, Nevada and Oregon. AGA President Geoff Freeman says only 3 percent of the $10 billion the group predicts will be wagered on the games will be done through legal Nevada sports books.