Charles and David Koch

Estimated net worth: $100 billion

How they made they money: Koch Industries, based in Kansas, is the country's second-largest private company. It began as an oil refinery and has since expanded into many other fields.

Who they're backing: The Koch brothers have historically stayed neutral in the presidential primaries, but that may change this time around. David Koch appeared to tip his hand Monday at a GOP fund-raiser by predicting that Scott Walker would be the nominee, The New York Times reported. But he backtracked almost as soon as the news got out, insisting he wasn't endorsing the Wisconsin governor. Brother Charles then told USA Today that five Republican contenders could get their support: Walker, Jeb Bush, Ted Cruz, Rand Paul, and Marco Rubio. But he said the family might contribute to multiple candidates before settling on a single pick.

Sheldon Adelson

Estimated net worth: $29.1 billion

How he made his money: Casinos

Who he's backing: Formally, no one yet. But he's repeatedly been linked to Rubio in recent weeks. Politico reported that Adelson has told friends that the first-term Florida senator is "the future of the Republican Party." Adelson is also pleased with Rubio's strong support for Israel. Still, the billionaire is hosting several GOP hopefuls this weekend in Las Vegas, at an annual event often described as an "audition" for his support.

Foster Friess

Estimated net worth: $530 million as of 2012, according to the Wealth-X research firm

How he made his money: Mutual-fund investing

Who he's backing: Friess is boosting Santorum again, but the bigger questions are whether the former Pennsylvania senator will even run, and if he does, how competitive he'll be. After faring surprisingly well in 2012, he is preparing for another try, but he has not drawn much attention as the newer and bigger names in the race.

Paul Singer

Estimated net worth: $1.92 billion

How he made his money: Hedge funds

Who he's backing: Like Adelson, Singer has also been linked to Rubio. But the hedge-fund billionaire who backed Mitt Romney and Rudy Giuliani in the last two campaigns "has not yet settled on any one candidate," a spokeswoman told me.

Robert Mercer

Estimated net worth: Unknown, but he reportedly earned $100 million or more annually between 2011-2013

How he made his money: Renaissance Technologies, a hedge-fund company

Who he's backing: Ted Cruz. Mercer is Cruz's big catch so far. According to The New York Times, he is "believed to be the main donor behind" four super PACs that have bankrolled the Tea Party star's early campaigning. Like Koch, however, the lower-profile Mercer has the potential to be a liability: His hedge fund has been under investigation for owing as much as $6 billion in unpaid taxes.