Billionaire casino magnate Sheldon Adelson is poised to donate close to $100 million this election cycle, with much of that total coming in untraceable “dark money” to conservative groups—a massive amount that could help decide which party controls the Senate next year.

Several of the casino mogul’s largest checks, in the mid-seven to low-eight figure range, are being sent to a quartet of conservative nonprofits that under IRS rules can mask donors’ names, say three GOP operatives and donors familiar with his contributions.

Adelson is focused heavily on helping the GOP capture the Senate, and by writing mega checks to politically active nonprofits, he can stay under the radar while still lavishing tens of millions of dollars groups that help Republicans get elected.

According to GOP sources, the most prominent dark-money outfits that have received—or are expecting—Adelson’s biggest checks are Americans for Prosperity, which was founded by the billionaire brothers Charles and David Koch; Crossroads GPS, which was cofounded by strategist and informal Adelson advisor Karl Rove; the conservative pro-Israel Republican Jewish Coalition; and the U.S. Chamber of Commerce.

These dark-money entities collectively snared tens of millions in 2012 from Adelson, and all have made courting the sometimes mercurial 81-year-old tycoon a top priority, including by making visits to his Las Vegas Sands casino hotels this year. In 2014, these outfits have made winning the Senate their top priority, and are in the midst of spending tens of millions on ads in battleground states such as Arkansas, Colorado, Iowa, and North Carolina to help GOP candidates.

Boasting a net worth that was recently pegged by Forbes at $31.6 billion, Adelson is no newcomer to making humongous political wagers: In the 2012 elections, Adelson donated close to $150 million, much of which went down the drain as Republicans failed to win back the White House or the Senate. Of that total spending, some $93 million went to super PACs and committees that must reveal their donors.

In a 2012 interview, Adelson said the main factor driving his big donations was fear that a second Obama term would produce “vilification of people that were against him.” Adelson also alluded to a still-ongoing federal probe into allegations that the Sands violated a U.S. law by bribing overseas officials to win business in Macau, where the company owns several lucrative casinos.

Although Adelson is still pouring millions into committees that do have to disclose donors, his dark-money footprint is expected to increase substantially this year. GOP operatives say that Adelson’s donations in 2014 seem poised to tilt decidedly toward dark-money outfits, a strategy that reduces the risk of his big checks becoming issues in tight Senate contests.

It’s little wonder that lots of GOP operatives and outside groups are banking on Adelson’s bulging checkbook to boost their election prospects this fall. Republicans view Adelson as an invaluable asset to counter outside Democratic groups with ties to Senate Majority Leader Harry Reid, and also liberal billionaire Tom Steyer, who’s committed to spending $50 million of his own funds to make climate change a key issue in seven Senate and gubernatorial races.

“With all the money that Democrats will have in Senate races from Harry Reid’s organization and Tom Steyer’s organization, it’s critical that pro GOP groups be well funded,” said longtime GOP political operative Charlie Black. “Sheldon Adelson is expected to be the single largest giver to those committees so his role is very important.”

Adelson is known for being a hands-on donor who makes decisions carefully. Sources say he seems bullish about the GOP’s chances of winning the Senate, noting that he has met personally with some of the leading candidates—including Rep. Tom Cotton, who’s running against Sen. Mark Pryor in Arkansas, and Rep. Cory Gardner, who’s looking to unseat Sen. Mark Udall in Colorado. And with just over two months before the midterm elections, sources note that Adelson-backed ad blitzes and get-out-the-vote efforts by dark-money groups could prove instrumental in helping the GOP pick up the six Senate seats it will need to take control of the chamber.

Republican donors and operatives familiar with the casino owner say that Adelson is also ponying up big bucks to help governors and House members, as well as favorite causes like pro-Israel groups and anti-union drives. This election cycle, Adelson’s larger public donations include $2.5 million to the Republican Governors Association. He gave the same amount to a group in Florida that’s fighting an initiative to legalize medical marijuana. In tandem with his wife and other family members, Adelson has kicked in hundreds of thousands to the Republican National Committee, plus the GOP’s House and Senate campaign committees.

Last year, Adelson also launched a high-stakes lobbying drive to enact legislation that would ban Internet gambling, an industry that competes with his casinos. Although Adelson has couched the issue in moral terms—citing the risks that Internet gambling poses to children—the legislation would also help his casinos fend off an online threat to their bottom line. South Carolina Sen. Lindsey Graham introduced a measure to ban Internet gambling just months after his campaign got a $15,000 contribution from the casino mogul, who also hosted a Vegas fundraiser for Graham. “I’m willing to spend whatever it takes” to stop online gaming, Adelson vowed in a Forbes interview.

Although a longtime GOP sugar daddy, Adelson’s public profile has soared since the 2010 Supreme Court ruling in the Citizens United case that ditched decades of campaign finance law and opened the door for corporations, individuals and unions to write unlimited checks to outside groups directly advocating for or against a candidate.

Since Citizens United, Adelson has been aggressively cultivated by streams of high-powered Washington visitors—and others—looking for fat checks in the last two elections. This year’s competition for Adelson’s wallet has been intense and involved new twists as rival groups who share many of the same goals vie for the most loot.

A spokesman for Adelson declined to comment on this year’s donations. But Andy Abboud, Adelson’s top political aide, has denied that Adelson planned to spend $100 million on Senate races, saying that “there is no set budget for this cycle. More importantly, any group that meets with us and leaks any information true or untrue gets cut off.” CNN first reported the $100 million spending figure this summer and suggested that it was Senate-related.

Although Adelson’s publicly disclosed donations to date are far short of $100 million, sources say he should end up near that figure through his dark-money operations.

Americans for Prosperity, the flagship of the Koch brothers’ far-flung donor network, is expected to spend close to $125 million this year, and seems to be one of the casino tycoon’s favored dark-money outfits this cycle. Adelson has been increasing his ties to AFP and the Koch network since 2012, the year Adelson attended his first Koch donor conference. These hush-hush confabs are typically held twice a year at posh resorts, and are attended by a few hundred rich conservatives, along with a select group of GOP stars from Congress and the states.

Adelson chipped in at least $15 million to AFP in 2012, say operatives, and has a few links with the grassroots advocacy group that was founded in 2004 by the Koch brothers. Famously known for his hawkish Middle East views, Adelson has nonetheless bonded with the libertarian-leaning Koch network—which is often critical of excessive Pentagon spending and foreign entanglements—on a range of domestic issues like lower taxes, less regulation, and broadly shared commitments to other GOP principles.

Those ties appear to be growing stronger. Adelson aide Andy Abboud attended at least one of this year’s semi-annual Koch conferences for its wealthy allies and is quite active with AFP, according to two conservatives familiar with the Koch network. At the January Koch conference in the Palm Springs area, Abboud led a discussion that drew many wealthy donors, say two attendees.

“Andy is very active with AFP,” one donor who went to the Koch conference told me, adding that he expects Adelson’s total funding of AFP will be higher than in 2012.

Other signs of the closer ties between Adelson and the Koch conservative network have been noticeable this year. Tim Phillips, the president of AFP, flew into Las Vegas in late March to attend the annual spring event for the Republican Jewish Coalition that was held at Adelson’s Sands. According to RJC members there, Phillips was a guest of AFP Ohio leader Eli Miller, an RJC member and an ex-aide to House Speaker John Boehner. And Adam Stryker, who just a few years ago was part of Adelson’s government affairs team at the Sands, is now chief technology officer for the AFP Foundation, a post he assumed this year after doing a stint as head of the AFP’s Nevada chapter.

Besides AFP, Adelson is expected to give mid-seven figure or eight figure checks to Crossroads GPS and its super PAC affiliate, American Crossroads. Both groups were co-founded by GOP strategist Karl Rove, who the casino owner has relied on as a political confidante in the last few elections and maintains strong ties with. This summer, Rove spent a few hours in Las Vegas with Adelson and his wife, Miriam, according to conservatives familiar with the matter.

In 2012, Adelson and his wife late in the campaign cut checks totaling $23 million to American Crossroads. Earlier in the year, according to conservative sources, Adelson gave close to $20 million to Crossroads GPS.

Another big winner is expected to be the Republican Jewish Coalition, a conservative pro-Israel group. Adelson has long been an RJC board member and its leading underwriter, according to two other board members. The RJC, say the sources, is expected to spend a record amount on political activities this year—most likely north of $20 million—with the bulk of the funds coming from Adelson.

The RJC will be spending and Adelson will be donating “significantly more than in the past,” one board member told me. Some of that total is likely going to be transferred to other dark-money groups, a path the RJC and others like Crossroads GPS have adopted before to adhere to IRS guidelines that say such groups can’t spend the majority of their funds on politics. In 2010, for instance, the RJC sent a $4 million check to Crossroads GPS.

Adelson’s fervent backing of Israel, and his strong opposition to a two-state solution for the Israeli-Palestinian crisis, have driven his financial support for several conservative pro-Israel groups—but the Republican Jewish Coalition has reaped the biggest dividends.

For several years now, Adelson’s Sands has hosted spring RJC meetings that draw big-name GOP leaders like Reince Priebus, the RNC chairman. In a new twist for the RJC and Adelson, this year’s RJC bash featured three governors—John Kasich of Ohio, Scott Walker of Wisconsin and Chris Christie of New Jersey—and Jeb Bush, the former Florida governor. All are potential contenders to head the GOP ticket in 2016, and all went to great lengths in their public remarks at the event—or privately with Adelson—to show their fealty to Israel in an attempt to impress the casino magnate.

And the U.S. Chamber is also in line to reap a multimillion-dollar check from Adelson this year, say GOP sources, as it did in 2012. The powerful business lobby held an event at the Sands this summer for its elite Public Affairs committee, a group of some 70 Chamber backers and other allies, which includes Adelson aide Abboud. The committee is charged with overseeing the Chamber’s political endorsements and other election-related activities, and Abboud, not surprisingly, was the host for the Chamber event at the Sands.

If Adelson’s hefty political bets are successful, the GOP will have a good shot at winning their election jackpot: control of the Senate, and the power that comes with it.