Republican White House hopefuls are working to win over a handful of GOP rainmakers who could bankroll their candidacies through a crowded and expensive primary season.

With former Florida Gov. Jeb Bush locking down a number of key establishment donors already, it's more critical than ever for other players to make inroads with the top undecided GOP moneymen. 

Here's a look at the biggest names still on the board.

The Koch Brothers

A network of 17 conservative groups backed by the billionaire brothers Charles and David Koch plans to spend nearly $1 billion in the 2016 campaign cycle — more than double what the groups spent in 2012.

The wealthy donors haven’t said yet whether they’ll wade into the Republican primaries, though. The duo stayed out of the primaries in 2012, and any meddling in the strong and diverse field of 2016 candidates could split conservatives and potentially saddle the groups with embarrassing losses.

Still, the breadth and depth of the Koch network is such that there could be some overlap between candidates, potentially buoying a handful of GOP contenders during the primaries, and then later go all-in to back the eventual nominee.

Sheldon Adelson

The casino mogul has a fortune estimated at roughly $30 billion, and he hasn’t been shy about showering it on his political favorites.

Adelson was far and away the biggest giver to outside groups and super-PACs in 2012 when he dropped nearly $100 million.

He almost single-handedly funded Newt Gingrich’s upstart campaign during the 2012 Republican primaries, giving about $15 million to a PAC associated with the former Speaker. Adelson then shifted his resources to support Mitt Romney once the former Massachusetts governor secured the GOP nomination.

The 2016 GOP contenders are already jockeying for position in the shadow “Adelson primary.” Nearly the entire field of the potential GOP candidates seems to have some connection to the billionaire, further intensifying speculation over who he’ll back.

But the early wooing could be futile — Bloomberg reported this week that Adelson won’t get involved in the primaries until “well into 2016.” 

That news came as a shock to many, as Adelson has a reputation for spending early and often. If he waits for the first quarter to pass, the field will already have been narrowed by the carve-out states and his influence could be moot. 

Foster Friess

Friess bankrolled Rick Santorum’s upstart campaign in 2012, and he told the Washington Post last month he’ll do so again if Santorum runs in 2016.

But the former Pennsylvania senator is buried in a crowded field and another upstart White House bid is no guarantee. If Santorum foregoes another run or falters early, there are a handful of strong social conservatives that could compete for Friess’ money.

Former Arkansas Gov. Mike Huckabee, retired neurosurgeon Ben Carson, and Texas Sen. Ted CruzRafael (Ted) Edward CruzSenators introduce bill to overhaul sexual harassment policy Greatest risk to the Republican majority? Rising interest rates GOP Senate primary heats up in Montana MORE all seem to fit the Friess profile.

The Harold Simmons estate

Simmons, a Dallas billionaire who made a fortune off his pharmacy chain, passed away in late 2013.

Before that, he was the second largest contributor to outside groups and PACs during 2012. According to the Center for Responsive Politics, Simmons dropped nearly $27 million that cycle, trailing only Adelson.

Simmons supported super-PACs associated with Gingrich, Santorum, and former Texas Gov. Rick Perry early on. He later gave generously to a super-PAC backing Romney.

Simmons left his estate to his wife Annette, and she hasn’t said publicly whether she intends to continue her late husband’s tradition of political giving. A family lawyer did not return a request for comment.

Still, there are early signs that Annette could be a player. In December, she gave a small donation to Florida Sen. Marco RubioMarco Antonio RubioAdministration works to assuage critics over ZTE deal Hillicon Valley: Judge rules Trump can't block Twitter users | ISIS content finds a home on Google Plus | Rubio rips ZTE demands as 'terrible deal' | Bill would protect kids' data Overnight Finance: Trump eyes 'different structure' for China trade deal | Trump mulls auto import tariffs | Banks get green light to offer short-term loans MORE’s campaign fund, according to the Center for Responsive Politics.


“That’s obviously a place some candidates will look, but they might not find much,” said Texas-based GOP strategist Matt Mackowiak.

The Bob Perry estate

Perry, who famously funded the “Swiftboat” ads against then-presidential candidate John KerryJohn Forbes KerryJohn Kerry to NYU Abu Dhabi: We can't address world problems by 'going it alone' Juan Williams: Trump's dangerous lies on Iran Pompeo: US tried, failed to achieve side deal with European allies MORE in 2004, also died in 2013.

The Houston homebuilder finished third on the Center for Responsive Politics list of biggest givers to PACs and outside groups in 2012, trailing only Adelson and Simmons. He pumped nearly the entirety of the $24 million he spent in 2012 into groups supporting Romney’s campaign.

However, one Texas Republican told The Hill it appears that the Perry family has “shut a lot of the political bidding down” after he passed away. 

Joe Ricketts

Nebraska’s Joe Ricketts founded TD Ameritrade and he bought the Chicago Cubs for his family to run.

Ricketts gave bulk of the $13 million he spent in 2012 to a small-government group called the Ending Spending Action Fund, a popular group among big donors on this list.

Now, there are rumors that Ricketts could be a player in the 2016 GOP primaries. He’s believed to be a backer of Scott Walker, according to one GOP strategist in Nebraska. 

Ricketts was one of several out-of-state billionaires who gave to the Wisconsin governor’s recall election. Walker later campaigned on behalf of his son, Pete Ricketts, in his successful campaign for Nebraska governor.

Fred Eshelman and Art Pope 

Eshelman was quietly one of the biggest givers to GOP causes in 2012. He gave nearly $7 million to RightChange, a group that seeks to elect conservative lawmakers.

Eshelman isn’t flying under the radar anymore. In 2014, he sold his drug development company and promptly made a $100 million to his alma mater, the University of North Carolina at Chapel Hill. 

Republican strategists in the state expect the Republican contenders will head down to the Tar Heel State to court Eshelman and Art Pope, the wealthy chairman and CEO of a discount store chain. 

Pope’s donations to Republicans, and his political activism, are widely credited with helping turn the North Carolina legislature in favor of Republicans.

“Any Republican that’s raised money in North Carolina knows about Art and Fred,” said Carter Wrenn, a GOP strategist in the state.

Paul Singer

The New York hedge fund manager was an afterthought in 2012, but in 2014 he emerged as the biggest-spending Republican, according to the Center for Responsive Politics.

Singer gave more than $9 million to a variety of conservative PACs in the last cycle, ranging from American Crossroads and Ending Spending Action Fund, to his pro-gay marriage PAC American Unity.

That last part makes him a wildcard within the GOP field — he’s actively pushed for Republicans to embrace gay marriage, though there doesn’t appear to be a major candidate that shares his views on the subject.

There’s one outsider who is the exception – former United Nations ambassador John Bolton is considering running for president, and he supports gay marriage. In 2014, Singer gave $25,000 to the John Bolton Super PAC.

Bob Mercer

Mercer is the head of a New York-based multi-billion dollar high-frequency trading fund. 

In 2012, he was a top 10 political donor, giving about $5.4 million to conservative causes. In 2014, he broke into the top five, giving roughly $8 million to conservative causes, more than every Republican on this list except for Singer.

According to a Bloomberg analysis, Mercer’s donations through is family foundation run the gamut from Koch-backed groups, to the George W. Bush Foundation, to a group backing candidates that oppose abortion rights, and one that works to refute the connection between human activity and global warming.

Peter Thiel

Thiel made his fortune as co-founder of PayPal. He was also an early investor in Facebook, and his net worth is in the billions.

Thiel largely stayed on the sidelines in 2014, but in 2012 he pumped millions into a super-PAC supporting  then-Texas Rep. Ron Paul’s doomed presidential campaign. That has political watchers wondering if he’ll do the same for Sen. Rand PaulRandal (Rand) Howard PaulOvernight Energy: Reporters barred from Day 2 of EPA summit | Dems blame Trump for gas price increases | Massachusetts to get new offshore wind farm Senate Democrats look for traction on gas prices GOP Senate primary heats up in Montana MORE (R-Ky.) in 2016.

The younger Paul has set up an office in Silicon Valley in an apparent play for some of the tech sector’ biggest donors.