NEW YORK (Reuters) - About 47 million people - one in five American adults - are expected to bet a combined $8.5 billion on “March Madness,” the annual men’s college basketball tournament, a new report said on Monday.

Mar 17, 2019; Nashville, TN, USA; General view of pyrotechnics in the arena prior to the championship game between the Tennessee Volunteers and the Auburn Tigers in the SEC conference tournament at Bridgestone Arena. Mandatory Credit: Jim Brown-USA TODAY Sports

A plurality of bettors - 29 percent - favor Duke University’s Blue Devils to win, according to a report from the American Gaming Association (AGA), a casino industry group.

The National Collegiate Athletic Association’s tournament to determine the Division I men’s basketball champions begins on Tuesday and ends April 8 in Minneapolis.

This year is the first time the tournament will be held since a U.S. Supreme Court ruling in May 2018 allowed states to legalize, regulate and tax sports betting.

Eight states now offer legal sports wagers, including Nevada, which was never subject to a ban.

More than $5.9 billion has been bet on sports in those eight states since the court decision, the AGA said in its report.

As the nascent legal U.S. sports betting industry expands, major events like the NCAA’s March Madness are providing first glimpses of how many betters may want to move from illegal to legal wagering, and how much money casinos, racetracks and bookmakers stand to make in the years to come.

Forecasts had suggested Americans would wager $325 million this year on another traditionally huge betting event, the Super Bowl.

But the two biggest state markets so far - Nevada and New Jersey - fell short. Nevada handled just $146 million of legal bets, an 8 percent drop from the previous year’s record $159 million.

A report from Eilers & Krejcik gaming analysts on Friday estimated that if all 50 U.S. states had legal online sports betting, sportsbooks would handle $15.2 billion of total wagers just for March Madness alone, grossing about $1.2 billion of revenue.

By March of 2023, as many as 39 states could have legal sports betting, Eilers & Krejcik found.

As for this year, March Madness will likely generate $4.6 billion of wagers from 40 million people betting with friends and colleagues through a total 149 million brackets, the AGA said.

The remaining $3.9 billion of wagers will come mostly by way of illegal offshore websites and bookies, though 4.1 million people will also place legal bets through licensed casinos and sportsbook operators.

“These results indicate there’s still work to do to eradicate the vast illegal sports betting market in this country,” said AGA Chief Executive Officer Bill Miller in a statement.