Senate Minority Leader Bruce Tarr wants to authorize the state's Gaming Commission to grant licenses to existing casinos to offer online betting. Massachusetts would become the fourth state to do so.

Supporters say Tarr's proposal will help a fledgling casino industry capture a larger share of gambling dollars while drumming up even more tax revenue for cash-strapped state and local governments.

"We don't want them to be undermined by other states that allow it," said Tarr, R-Gloucester, sponsor of the bill.

The legislation doesn't allow new casino licenses, he said. Nor does it sanction casinos to offer a venue for fantasy sports -- an unregulated, multi-million dollar industry that is already attracting state scrutiny.

Massachusetts is expected have three regional resort casinos and a slots parlor up and running in the next several years, including a 24-story resort along the Everett waterfront operated by Las Vegas casino mogul Steve Wynn.

Penn National Gaming has spent $225 million to expand a harness racing track in Plainville into a slot parlor, which opened in June. MGM Resorts International is developing a $800 million resort casino in Springfield, which is set to open next year. And several casino operators are competing for a license in the South Shore region.

But casino operators are divided over web-based gambling, said Chris Moyer, a spokesman for the American Gaming Association.

Some casinos want to tap the online gaming market, he said, while others worry that it will keep customers from visiting their facilities and spending money on food, entertainment and other amenities.

"There's varying interest from the industry," he said. "There's some who view it was a way to attract more people, and others who see it as possibly decreasing the number of customers to land-based casinos."

Representatives for MGM, Penn and Wynn didn't respond to requests for comments.

A U.S. Justice Department ruling in 2011 gave states authority to offer Internet gambling to residents. Only three -- Nevada, Delaware and New Jersey -- have taken advantage of the ruling.

New Jersey Gov. Chris Christie, a 2016 Republican presidential candidate, approved online gambling in the Garden State two years ago as part of an effort to prop up Atlantic City's struggling casinos.

New Jersey officials are also seeking to legalize fantasy sports betting at casinos, but that proposal has been tied up in court challenges from college and professional sports franchises that oppose legalized wagering on their games.

Meanwhile, Congress has been debating a bill -- backed by some casino operators -- that forces those states to end their online gambling operations unless they accept federal sanctions.

For states that allow gambling - online or otherwise - big money is at stake.

Massachusetts will take 49 percent of gambling revenues from the slots parlor and 25 percent from the casinos once they open.

Overall casinos are expected to drum up more than $300 million a year in taxes -- money for education, local governments and other budget items, state officials say.

But the Bay State's casinos will enter a region already flush with well-established gambling facilities in Connecticut, Rhode Island, Maine and New York.

That has state officials concerned about a saturation of the regional market.

Plainridge Park Casino reported $12.9 million in revenue in October, down from $18 million in July. It expects to generate about $176 million in its first full year, roughly $24 million less than the state’s $200 million projection.

While online gambling has been heralded as a way to reinvigorate a sluggish casino industry and provide more state revenue, groups that work with compulsive gamblers say it could exacerbate existing social ills.

A recent study by the Cigna Foundation suggested that 7 1/2 percent of Massachusetts residents -- from 350,000 to 430,000 adults -- are considered "at-risk" gamblers. The study found adolescents are even more vulnerable.

"Our concern is that this would make gambling easier and more accessible," said Krystle Kelly, a spokeswoman for the Massachusetts Council on Compulsive Gambling. "Are we making it easier for people to fall deeper and deeper into trouble?"