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 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, and a ruling that legalizes sports betting nationwide could provide new revenue opportunities for cash-strapped state governments, as well as casino companies.

The group found 54 million people — or about a quarter of the U.S. adult population — participated in a sports betting pool last year, spending $18 billion on entry fees. That includes 24 million who filled out basketball brackets pools and spent $2.6 billion on entry fees.

Freeman said only 3 percent will be bet legally.