Casino billionaire Sheldon Adelson spent more on the 2012 federal elections than any other donor, putting up about $90 million of his family's money.   

His willingness to make a huge political investment helps explain why his signature group, the Republican Jewish Coalition, has attracted all of the major GOP presidential candidates to its Thursday forum in Washington -- even though the man himself won't be among the 700 people in attendance.   

From 9 a.m. to 5 p.m. EST, from top-polling Donald Trump to barely registering Jim Gilmore, the candidates will give 30-minute speeches and take questions from Jewish activists. Most have addressed the coalition at one or more of its past events, including one earlier this year in Las Vegas.   

The coalition, for which Adelson is a major funding source and board member, has held presidential forums in almost every election year since 1988. Apart from the debates, this is one of the only 2016 events that has attracted every single candidate to one room.   

Matt Brooks, RJC's executive director, said that unlike the debates -- where candidates often have just a few seconds to respond to questions -- Thursday's forum gives plenty of time for deep explanations of policy.   

Attendees, he said, "are looking to see who passes the commander in chief threshold. They want to know what's in their hearts and guts on these weighty issues."   

While there's no chance Thursday for the candidates to personally interact with Adelson, many of them have already met with him. Another opportunity is less than two weeks away, when the Dec. 15 Republican debate will take place at the Venetian, a Las Vegas casino and hotel that's part of Adelson's international entertainment empire.   

Each of the candidates is strong on the issues that concern Adelson the most, chief among them protection of Israel, said his political adviser, Andy Abboud.   

"He has no plans now, or in the immediate future, to get involved in the primary," Abboud said, noting that Adelson won't be at the forum because he's on a personal trip.   "The Adelsons are generally pleased with all of the Republican candidates and feel that the primary process will work its way out," he said.   

Still, the specter of Adelson's nod looms large for the GOP field.   His family members began giving to a super PAC helping former House Speaker Newt Gingrich's 2012 presidential bid in late December 2011, fundraising records show, and Adelson gave the group $5 million around the time of the Jan. 3 Iowa caucuses.   

The family put more than $15 million into Gingrich's run -- donations that proved critical to extending his time in the race for the nomination. Unlike the candidates' official campaigns, super PACs can accept donations of any size.   Adelson had a long and friendly relationship with Gingrich, but is taking a more pragmatic approach to the 2016 GOP contest.   

"It's important to him that campaigns show that they can garner their own resources, build their own ground game and effectively mount a campaign that can win in the fall," Abboud said.   

The RJC's presidential forum offers candidates the chance to impress other wealthy donors, too. And many of the super PACs aligned with candidates, including former Florida Gov. Jeb Bush's Right to Rise, are hosting events Thursday night.   Many of the candidates have been eager to portray themselves as close to Adelson.   

Trump, who has said he is so wealthy that he doesn't need the help of donors like Adelson, said in an Oct. 13 interview on Fox News, "I like Sheldon a lot. He's been a person I've known over the years. We have a very good relationship."   

On the eve of the gathering, Trump declared himself a "big, big fan" of Israel, and said chances for a lasting peace rest with the Jewish state. "A lot has to do with Israel and whether or not Israel wants to make the deal -- whether or not Israel wants to sacrifice certain things," he said.   

Florida Sen. Marco Rubio has said in interviews that he regularly talks with Adelson, and the two had dinner in Las Vegas in early October. Texas Sen. Ted Cruz received a standing ovation at the RJC's conference this April and had a private meeting with Adelson.   

A year earlier, Ohio Gov. John Kasich addressed the same crowd -- but his remarks seemed tailored to an audience of one: Adelson. He repeatedly made eye contact with the billionaire and concluded by thanking "Sheldon" for inviting him to attend.