As GOP presidential candidates take the debate stage tonight at an extravagant Las Vegas hotel, they will once again compete for voters in an increasingly unpredictable race. But they are also vying for the attention of the man who owns the building — and no candidate has worked harder than Florida's Marco Rubio.The U.S. senator has avidly courted casino magnate Sheldon Adelson, sitting down with him privately numerous times, including a dinner in Washington weeks before launching his campaign in April, and checking in regularly by phone to talk about Israel and the campaign.In October, Rubio held a campaign meeting at Adelson's Venetian hotel, site of the debate.And Rubio has consistently championed Israel in speeches on the Senate floor while also pushing legislation aimed at supporting the cause.The "Sheldon Adelson primary" winner will land a prize worth tens of millions in financial support. Adelson, chairman and CEO of Las Vegas Sands Corp., gave $15 million to a super PAC supporting Newt Gingrich in 2012, helping unleash TV attack ads on Mitt Romney's business record. He then gave millions to support Romney.All told, Adelson and his Israeli-born wife, Miriam, spent $93 million that cycle, by far the No. 1 individual donors.This time, Adelson, whose worth is valued at somewhere between $20 billion and $30 billion, reportedly wants to throw his weight behind a more electable candidate, and he's prepared to spend even more. "I don't cry when I lose," he told the Wall Street Journal in 2012. "There's always a new hand coming up."Rubio has benefited from an outside group that has run TV ads featuring his hawkish foreign policy views, including a vow to tear up the Iran nuclear deal, which Adelson loathes. Rubio is also backing legislation Adelson is pushing to crush an expansion of online gambling, which threatens his global casino empire.Much of Rubio's supposed favor has been conveyed by people who are close to Adelson, not Adelson himself, who rarely talks to the news media.Among the mentioned attributes are Rubio's foreign policy outlook, his communication skills and appeal to a broad range of GOP voters. Recent polls show Rubio, 44, would be one of the strongest challengers to Democrat Hillary Clinton. Adelson also likes that Rubio is the son of immigrants.Adelson is not as well-known publicly among the uber-donors, such as the Koch brothers, but most of the candidates are angling for his support. It would be a major coup for Rubio, who has already gained support from Miami billionaire Norman Braman and New York billionaire investor Paul Singer, among others.Adelson, 82, grew up poor in the Boston area, the son of an immigrant cab driver, and was bullied by Irish kids for being Jewish. By age 12, he had his own business selling newspapers. (Today he owns Israel Hayom, a Tel-Aviv-based newspaper that has featured Rubio prominently this year.) By the late 1970s, Adelson was running a popular computer trade show in Las Vegas and eventually got into the casino business. A one-time Democrat, he became a Republican in the 1990s, seeing his old party as not supportive enough of Israel. Adelson is a critic of unions but moderate on social issues and supports stem-cell research and immigration reform.Throughout, Adelson has been an unwavering supporter of Israel."He's someone who believes in what he's doing for reasons beyond just trying to have an in with politicians later," said Sarah Bryner, research director at the Center for Responsive Politics, which tracks campaign fundraising.Who will grab Adelson's support has become a speculative game among reporters and political insiders. Politico reported in October that Adelson, who attended the Venetian gathering, was prepared to get behind Rubio and an endorsement was imminent. Since then, other news outlets have said Miriam Adelson is fond of Texas Sen. Ted Cruz.Rubio and Cruz have gained attention in the race, with Cruz leaping ahead in some polls, and they are going after each other on national security and immigration policy — a clash likely to be played up on the debate stage.A spokeswoman for the Las Vegas Sands did not return a phone message seeking comment. Rubio's campaign also declined to comment, referring to comments Rubio made in October on Fox News in which he called Adelson a "great American."One candidate unlikely to win Adelson's support is Donald Trump. "Sheldon Adelson is looking to give big dollars to Rubio because he feels he can mold him into his perfect little puppet," Trump wrote recently on Twitter. Still, Trump made his own attempts to curry favor with Adelson, including phone calls in which he talked about living in a city with a large Jewish population and, according to the New York Times, noting that his daughter converted to Judaism.Rubio has insisted his conversations with Adelson have focused on policies that promote U.S. support of Israel.Adelson does have business interests, and earlier this year Rubio attracted attention when he signed onto a bill that Adelson is trying to get through Congress that aims to curtail online gambling in states, a threat to his casino empire. Though Rubio has talked about state's rights and avoiding picking "winners and losers," he has attributed his support for the bill to a feeling that the Internet has fewer safeguards to protect people from fraud and addiction."People buy into my agenda. I don't buy into theirs," Rubio, who lived in Las Vegas as a youth, told reporters in June, noting his opposition to expanded gambling from his time in the state Legislature.Tonight, Rubio will be back at Adelson's hotel casino, looking to seal the deal with the billionaire he has carefully courted."Rubio calls and says, 'Hey, did you see this speech? Did you see my floor statement on Iran? What do you think I should do about this issue?' " a September New York magazine story quoted an unnamed Adelson friend as saying. "It's impressive. Rubio is persistent."Contact Alex Leary at [email protected] Follow @learyreports.