Republican presidential contenders are landing big-money donors, but the biggest fish remain at large.

Marco Rubio made the latest splash last week when Norman Braman, a wealthy Miami auto dealer, pledged to contribute upwards of $10 million to a super PAC backing the Florida senator's potential presidential candidacy. It came amid a string of moves by Republican presidential prospects who are asking the party's top donors and fundraisers to commit to their campaigns. Jeb Bush has spent the last several weeks scooping up top GOP donors all around the country, including New York Jets owner Woody Johnson, who was a top fundraiser for Mitt Romney's 2012 campaign. Home Depot cofounder Ken Langone and former Texas Republican Party finance chairman Ray Washburne are helping Chris Christie raise early cash. And even long-shot presidential contender Rick Santorum has Foster Friess, a millionaire businessman, on his side as he considers a second bid.

Still, some of the most prominent Republican donors and bundlers from the past few elections have yet to commit to a 2016 presidential candidate. Here are seven of them.

Paul Singer: After Woody Johnson signed on to help Bush, Singer is one of the biggest remaining bundlers from Romney's 2012 campaign. Singer kept up his political giving habits during the last election, contributing more to super PACs in 2014—$10.5 million—than any other Republican in the country, according to the Center for Responsive Politics. Singer's staunch support for same-sex marriage sets him apart from the rest of the GOP donor community, so any candidate who shifts to the middle on that issue could win his backing. One of Singer's most recent donations went to Rubio's leadership PAC last December, but for now, he is keeping his 2016 options open.

Sheldon and Miriam Adelson: If you're a Republican seriously considering running for president, you've probably already met with Adelson—and not just once. The Las Vegas casino magnate, along with his wife, bankrolled a pro-Newt Gingrich super PAC and spent nearly $100 million during the 2012 elections. Just last week during his trip to Washington, Adelson shared dinner with Rubio, cochaired a fundraiser for Lindsey Graham's new political committee, and met with Rand Paul to assure him that he does not plan to spend money against him in the 2016 primaries. And while he won't oppose Paul, an Adelson aide recently told Bloomberg Politics that the mega-donor won't officially back a presidential candidate either until "well into 2016."

Charles and David Koch: The Koch brothers haven't intervened in White House nominating contests in the past, but there are signs that 2016 will be different. The Kochs' sprawling political network plans to spend a staggering $889 million during the 2016 election cycle, some of which could go toward the GOP presidential primary. The donors who comprise the network have already had the opportunity to personally size up four GOP White House hopefuls—Ted Cruz, Scott Walker, Paul, and Rubio—who were invited to their annual summit in California earlier this year. Rubio topped a straw poll of some of the conference's attendees, but it's still far from determined who—if anyone—will benefit from the Kochs' cash in the primary.

Joe Ricketts: While they may not have the name recognition of the Adelsons or the Kochs, the Ricketts family have established themselves as GOP power players in the Citizens United era. Leading the charge is Joe Ricketts, the founder of TD Ameritrade, who gave $13 million to super PACs during the 2012 election and another $9 million in the 2014 midterms. As for 2016, his support is still of for grabs, and he's listening to offers. Last month, Ricketts hosted officials from seven super PACs supporting potential GOP presidential candidates, along with several other top Republican donors, at his home in Wyoming. But some White House hopefuls, like Cruz, Walker, and Mike Pence, already began courting the Ricketts family last year when Joe's son, Pete, was running for Nebraska governor.

Spencer Zwick: Romney's former national finance chairman, Zwick is another major holdout from the last White House campaign. While you probably won't see Zwick writing big checks to candidates or outside groups in 2016, he does hold the key to Romney's vast donor network, which just about every Republican presidential hopeful is vying for access to. Since Romney announced in January that he would not run for the White House a third time, Bush, Christie, Paul, Rubio, and Walker have all directly contacted Zwick. Zwick has said doesn't plan to team up with a candidate for some time, but he has recently spoken favorably of Bush and Rubio.

Robert Mercer: The co-CEO of the hedge fund Renaissance Technologies, Mercer has so far flown further below the radar than some of his counterparts as the GOP contest gears up, but his cash will also be highly sought after in the coming months. Mercer followed up his $5.5 million super-PAC effort in 2012 by shelling out another $9 million to GOP-aligned groups in 2014, including $2.5 million to Freedom Partners Action Fund, the super-PAC arm of the Koch brothers' operation. Where his money will go in 2016 remains to be seen.

Annette Simmons: Simmons is perhaps the biggest wild card on this list. She and her husband, Harold, who owned Contran Corporation, together donated more than $25 million to super PACs in 2012—an amount that was eclipsed only by the Adelsons. Harold Simmons, however, died in late 2013, so it is unclear to what extent Annette Simmons will use her family's fortune in the next presidential race. Simmons made a handful of contributions to Republican House candidates and party committees during the 2014 election, and, like Singer, she also cut a check to Rubio's PAC at the end of last year.

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