Who is flooding campaigns with secret millions?

Spinners and Winners

An infusion of millions of dollars, unlike anything we have ever seen before may now be the single biggest force in American politics. Some of the players are well-known, such as conservative activist and former George W. Bush adviser Karl Rove, and the U.S. Chamber of Commerce, a business interest group long active in politics.

This year, there are new players on the scene, so-called SuperPACs, that, unlike candidates or political parties, can accept an unlimited amount of money. For many of these groups who gives and why is a tightly-guarded secret.

"We can see the money going in, but it is very hard to follow what the political favors out are," says Bill Allison of the nonpartisan Sunlight Foundation. "I think that is the most concerning thing about all of this money going into the political process."

There are at least two dozen individuals who have already given more than $1 million each to these groups -- almost all Republicans. Spinners and Winners tried to track them down and ask why they are giving so much money.

Democrats play this game too, although with less money. Labor unions are expected to spend up to $400 million on campaigns this year, and Priorities USA, a pro-Obama group started by former White House aide Bill Burton, plans to spend another $100 million. They too are keeping many of their donors entirely secret.

ABC News' Gregory Simmons Lemos contributed to this report.

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