It's not just the office pool anymore. More than half of U.S. states now allow legalized sports betting, and in many cases it's as easy as opening an app on your smartphone and making a wager.

A whopping 17.6 million people are expected to place a bet on Sunday's Super Bowl online or in person at a sportsbook, according to the American Gaming Association.

It's not all profits for the operators, though. Sportsbooks are putting up their own money in the form of a massive advertising campaign targeting the millions of sports fans who will be glued to their TVs for the matchup between the Los Angeles Rams and the Cincinnati Bengals.

"TV sportsbook advertising has grown exponentially this year and in the last couple of years overall," Adam Candee, managing editor of the trade publication Legal Sports Report, told NPR.

Candee said the sudden growth of legalized gambling in states across the country has sportsbook operators jockeying for a piece of the huge new customer base — and the Super Bowl is their latest opportunity to sell themselves to potential bettors.

"It is happening in the here and now, moment to moment, as they compete for customers. That is because this is essentially a new industry that is rising up from the ground," he said.

Sports gambling could become a $37 billion industry by 2025, according to the investment management company ARK.

Legal sports betting has exploded over the past four years

In 2018, the U.S. Supreme Court struck down a ban on sports betting outside of Nevada, essentially giving the green light to states to allow sports betting if they chose to.

Many did. Thirty states and Washington, D.C., now permit legal sports betting and have active markets, and three more intend to launch legal sports gambling. (States are also reaping tens of millions of dollars in new tax revenue thanks to the flourishing industry.)

How you can bet on sports differs by state. Some restrict sports gambling to a few in-person casinos, while states at the other end of the spectrum permit anyone of legal age within their borders to place a bet online.

In just a few years, what was once a niche entertainment sector centered in Las Vegas has grown into a national mega-business that people can enjoy from their living rooms.

That has led to a spike in the number of people with gambling problems, the National Council on Problem Gambling has warned.

Still, Candee said major sports-betting outfits are now going all-in on advertising, and they're even willing to follow the business models of companies like Amazon that lost money in the short term in order to draw in a sprawling customer base that they hoped would remain for years.

"You have early movers who are heavily capitalized who are trying to acquire as much market share as they possibly can," he said.

The gambling industry, which includes sportsbooks, spent $488 million on advertising between November 2020 and November 2021, according to the research firm MediaRadar.

Caesars sportsbook recently announced it was spending $1 billion on marketing and acquisitions, Candee said, and companies such as DraftKings and FanDuel have lost money amid a major advertising push.

The sports-betting boom has also produced some online companies hoping to lure in unsuspecting players with misleading deals. On Thursday, New York Attorney General Letitia James warned residents of the state, which legalized sports betting last month, to be cautious of deceptive companies when choosing where to place their bets on game day.

The NFL used to oppose sports betting. Now the Super Bowl is one of the industry's marquee events

For years, the NFL and other major sports leagues opposed legalized sports betting, in part because of the possibility for cheating and match-fixing.

NFL Commissioner Roger Goodell said in a 2012 deposition that gambling was the No. 1 threat to the integrity of U.S. professional football, ESPN reported.

A lot has changed since then. The NFL has partnered with seven sportsbook operators, which allows them to buy in-game ads. The league's top-tier partners — Caesars, DraftKings and FanDuel — were able to integrate sports-betting content into the NFL's website and app.

And those companies are willing to spend millions of dollars during and in the lead-up to Sunday's Super Bowl.

Caesars and DraftKings will air national in-game TV commercials, while FanDuel will run pregame and local in-game ads, according to Legal Sports Report.

NBC announced it had sold out of all its in-game TV spots for this year's Super Bowl, some of which cost advertisers $7 million for 30 seconds of airtime.