Moves to allow casinos in Japan have failed for 15 years, but the chance of success has improved sharply, people involved in the effort say, thanks to political shifts that could open the world's next great frontier for high-roller gambling.

The public opposes casinos by 2-to-1 on concerns such as gambling addiction, yet insiders say political momentum has shifted in favor of the latest in a string of bills proposing to legalize them. Companies from Las Vegas Sands Corp. to domestic game-machine maker Sega Sammy Holdings Inc. stand to benefit if casinos come to what CLSA estimates could be a $40 billion annual market, the world's second-biggest after the United States.

"Integrated resorts will be a headline attraction for Japan's growth strategy," Prime Minister Shinzo Abe said in 2014, referring to projects that combine casinos with hotels, shopping and conference facilities. "We will continue to consider them from the viewpoint of how to attract people from around the world."