Rival casinos and online poker companies are counterattacking through the Coalition for Consumer and Online Protection. The group has signed up two Republican former House members, Michael G. Oxley of Ohio, who a decade ago led efforts to outlaw online betting and accused companies selling such games of “gobbling up victims in the United States,” and Mary Bono of California. Mr. Oxley, who retired from Congress in 2007 and now works as a lobbyist, said in an interview that he believed state-regulated online gambling was now the best hope of countering the rapid expansion of illegal online gambling.

“The world has changed dramatically in the last 10 years,” Mr. Oxley said. “I have come to the conclusion you can’t try to control the Internet and the like.”

Other hires include Jim Messina, President Obama’s former campaign manager, and Haley Barbour, a former governor of Mississippi and prominent Republican lobbyist, who helped Republican “super PACs” raise millions of dollars in the effort to beat Mr. Obama in 2012. Among the coalition’s arguments against a ban on online gambling is one that also figured in Republicans’ battle against Mr. Obama’s health care expansion: that it violates states’ rights.

Some in the industry worry that a brawl between its biggest players could threaten the painstakingly built image of family-friendly entertainment the casino resorts have worked to promote.

“It is unfortunate, when an industry undermines itself,” said Jan L. Jones, a former mayor of Las Vegas who is now the head of government relations at Caesars Entertainment, which is opposed to the bill. “This fight is tarnishing the entire industry. You just raise a whole specter of negativity that I think is unfortunate and inappropriate, after we have spent the last three decades with a message that gaming is just entertainment enjoyed by responsible adults.”

What is clear is that an enormous amount of money is at stake: Morgan Stanley, an investment banking firm that tracks the industry, has estimated that by 2020, online gambling could be worth $8 billion a year — about 12 percent of the overall current gambling revenues at commercial and Native American casinos nationwide.