It is hard to argue that casino-owner and Republican super-donor Sheldon Adelson is not a gambling man. But his apparently imminent endorsement of Marco Rubio, the Florida Senator hoping to take on Hillary Clinton in next year's presidential election, still appears something of a long shot. Trailing frontrunner Donald Trump by nearly 20 points, the 44-year-old Cuban-American hardly seems a sure bet for the Republican nomination.

Nonetheless, even while trailing in the polls, Mr Rubio has fought the famed "Adelson primary" relentlessly. He is said to phone the billionaire several times a month to update him on his campaign. For good reason: with an estimated net worth of $32 billion, Mr Adelson spent $100 million backing Republican candidates in 2012.

Certainly, after a cash-strapped summer, money now appears to be flowing into Mr Rubio's campaign. Last week, he won the endorsement of billionaire hedge fund manager Paul Singer, who, like Mr Adelson, serves on the board of the Republican Jewish Coalition.

But Mr Singer and Mr Adelson's backing for Mr Rubio does not simply reflect the presidential hopeful's hawkish foreign policy views - throughout the summer he trumpeted his opposition to the Iran nuclear deal - nor his self-described "unconditional support" for Israel. Such views are widely shared among those contesting the Republican nomination. Rather, it is an indication of the manner in which the Republican establishment is now rallying behind Mr Rubio, a one-time Tea Party favourite who, while remaining staunchly conservative, has inched towards the centre ground since his election to the Senate in 2010.

With Jeb Bush's campaign sinking under the weight of poor poll numbers and the candidate's lacklustre performance in last week's Republican debate, this trickle of support towards Mr Rubio may soon become a flood as the party's leadership and more moderate business wing attempts to prevent Mr Trump or his fellow outsider, the evangelical former neurosurgeon Ben Carson, from winning the nomination. As an Adelson associate told Politico in April: "He doesn't want the crazies to drive the party's prospects into the ground." Indeed, Mr Singer's endorsement last week pointedly suggested that Mr Rubio was the only candidate who can "navigate this complex primary process and still be in a position to defeat" Mrs Clinton next November.

With one year to go until the general election, such surveys should be viewed with huge caution, but polls pitting Mr Rubio against Mrs Clinton show the two candidates in a virtual tie. Mr Adelson believes the youthful Latino, the son of a bartender and hotel maid, is best able to address the principal reason for Mitt Romney's defeat in 2012: the GOP candidate's inability to reach beyond the party's base of older, wealthy white votes.

History suggests that to bet against Mr Rubio might be unwise. When, in May 2009, he announced he planned to run for Florida's vacant senate seat, he trailed the state's popular governor, Charlie Crist, by 30 per cent in the polls.

Within 18 months, Mr Rubio had dispatched Mr Crist, won the Republican nomination and a seat in the Senate, and sparked a wave of speculation about a future run for the White House.