Heading into the 2012 election, Las Vegas casino magnate Sheldon Adelson made the headlines scream when he announced his intentions to spend $100 million on Republican candidates. It seemed like an unfathomable amount of money for an individual, even an incredibly rich individual, to fork over in a single election cycle. Now, two years later, it's clear his nine-figure barrelhead was just setting the bar.

David and Charles Koch Getty Images (L) | Koch Foundation (R)

Earlier this year, that San Francisco hedge fund manager Tom Steyer talked up his own plans to spend $100 million in 2014 to support of environmentally friendly politicians, particularly those who oppose the Keystone pipeline. Then last week, Poltico reported that the Koch brothers' main political outfit, Americans for Prosperity, intends to spend $125 million on an all-out campaign to support conservative candidates.

What's next?

"As mind-boggling as that seems, a billion-dollar donor is only a matter of time," said Sheila Krumholz, executive director for the Center for Responsive Politics, a watchdog group that tracks political contributions. "The trajectory is only going in one direction, whether you are talking about the total costs of elections, which is on an inexorable climb—two steps forward, one step back." As Steyer and Adelson have demonstrated, megadonors are no longer cagily demurring about how much money they intend to spend. Quite the opposite: They are haughtily advertising it.

"I think it is a badge of honor," said Fred Davis, a Republican media consultant who has worked with major donors on outside-group campaigns.

"There is a star quality of having a big round number," said Krumholz.

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And now that liberals donors have more fully joined the Super PAC fray—after some initial angst over 2010 Supreme Court ruling that paved the way for unlimited contributions to outside groups—a true game of one-upmanship is afoot.

The intensifying megadonor arms race has been egged on of late by the battle for the Senate, which is very much up for grabs.

"I think to a large degree the reason you are seeing so much money is because the balance of power is real," said Rick Tyler, a former top aide to Newt Gingrich. During the 2012 election, Tyler served as an advisor for Winning Our Future, a pro-Gingrich Super PAC that received $20 million from Adelson.

That was roughly the same amount liberal billionaire George Soros dropped jaws with in 2003, in an unsuccessful attempt to boot George W. Bush from the White House. His failure had a cooling effect on his giving: In 2012, for example, he waited until the last leg of the presidential election before finally handing over $1.5 million to Democratic Super PACs, a relative pittance compared to the other side.

There was some hope among campaign reform types that Republican electoral failures in 2012 would halt the spending bonanza by Adelson, the Koch brothers, and their like-minded plutocrats.

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But that has proven wishful thinking.

"If you look at Sheldon's entire career, the guy started out selling newspapers. He clearly never gives up," said Tyler. "That is how he created his whole empire—by persistence and not giving up."

Adelson has indicated he will try to spend his money on more electable candidates, as opposed to spending less of it.

Bill Allison, editorial director of the Sunlight Foundation, said the current stable of megadonors are largely ideological and therefore more keen to doubling (or tripling) down—even in response to past nonsuccesses. Allison said he "wouldn't be shocked" if an individual donors drops $200 million in the 2016 election. And as we've come to see recently, even wild projections tend to fall short of reality.

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Davis said the arms race is not just being accelerated by donors trying to prove something, but by an industry of political functionaries drawing increasingly ambitious budget proposals.

"Do these (donors) generally say, 'My number is $125 million?'" said Davis. "No, there is somebody selling them on that investment, whether it is Karl Rove or some of the liberal groups—there is somebody out there getting their eye and catching their attention."