* Japan lawmaker says 50:50 chance casino bill will pass by June
* Garnering support from opposition Democratic Party proving hurdle
* Dragging out debate would be missed opportunity for Japan - MGM CEO
(Recasts, adding background on bill debate, comments from lawmakers, MGM CEO)
TOKYO, May 15 (Reuters) - Japan may not be able to pass a bill in the current parliament session, which ends next month, that would pave the way to legalising casino gambling, two lawmakers said on Thursday.
Delaying the bill could thwart global casino operators' ambitions to set up in one of the world's last untapped gaming markets by 2020, when Tokyo is due to host the Olympic Games.
Japan could become the third largest gambling destination after Macau and the United States, with annual revenue of over $40 billion, according to broker CLSA, and global casino executives have been visiting with increasing frequency, promising to invest billions of dollars in resorts they say would boost tourism and the economy.
Politicians from the ruling Liberal Democratic Party (LDP) and two smaller parties submitted a casino bill to parliament in December, with an aim to start debate in April or May and secure its passage by the end of the current session on June 22.
But Sakihito Ozawa, a casino proponent and a member of the Japan Restoration Party, told a casino industry conference in Tokyo that it was becoming increasingly difficult to get the bill passed with only five weeks of the current session remaining.
"I'd say the chances are 50:50," Ozawa told Reuters on the sidelines of the conference, which had been timed to coincide with the possible passage of the bill.
Takeshi Iwaya, a veteran LDP lawmaker who has been working to legalise casinos for more than a decade, told the conference he was still aiming to start debate on the bill this month, but acknowledged there were obstacles.
Iwaya said he had spoken with members of the New Komeito party, a junior coalition partner, and would be explaining the merits of casino resorts to the opposition Democratic Party next week in hopes of garnering their support.
He said lawmakers were behind their envisioned schedule, partly due to time spent on other legislation.
Passing the initial bill by next month is seen as crucial for the first casino to open in time for the influx of visitors for the Olympics. Jim Murren, CEO of MGM Resorts International , said a prolonged debate would damage Japan's prospects.
"To have a goal of opening a Japan resort by 2020 is a worthy, ambitious and achievable goal if there is proper focus," Murren told the conference. "If we are in debate for another couple of years, who knows, and it would be a tremendous missed opportunity."
(Reporting by Nathan Layne; Editing by Chris Gallagher, Miral Fahmy and Ian Geoghegan)