American Crossroads – a pro-Republican "super PAC" formed with help from Karl Rove, the former strategist for George W. Bush – has emerged as a powerful force in key U.S. Senate races this election season, dominating groups of its kind in fundraising and spending, according to a review of campaign finance data by ABC News.
The group is also the most prominent example of the new campaign finance dynamic created in the wake of two Supreme Court decisions earlier this year that lifted some limits on campaign spending and allowed corporations and interest groups to funnel money directly into political campaigns.
American Crossroads and more than two dozen similar groups -- known technically as independent expenditure-only committees -- can raise and spend unlimited amounts of cash from individuals, corporations, and unions with the sole purpose of running advertisements in support of specific candidates.
Unlike traditional PACs, or political action committees, they cannot give money directly to candidates or coordinate their efforts with individual campaigns.
American Crossroads has outspent the next group of its kind, the conservative Club for Growth, by more than double, $4.2 million, according to campaign finance filings through August.
It has also raised more than $31.6 million for the midterm election campaign, according to spokesman Jonathan Collegio, well on its way to a goal of $52 million by Nov. 5. Most of its cash has been raised since Labor Day and has not yet been reported to the Federal Elections Commission.
American Crossroads is "on the vanguard of this new wave of independent expenditure-only committees," said Dave Levinthal of the Center for Responsive Politics, which tracks campaign finance data. "They've been very aggressive. But we've got a whole lot of election left to go. These numbers are just a snapshot in time."
Some of the group's biggest TV ad buys have been in Missouri, where it has spent more than $700,000, and in Colorado, where it has spent more than $500,000, attacking Democratic U.S. Senate candidates and their records on government spending.
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American Crossroads has also aired ads in the Ohio, Illinois, Nevada, New Hampshire and Kentucky Senate races, pushing values of "individual liberty, limited government, free enterprise and strong national security," according to the group's website.
The group, which some have labeled the "shadow RNC," was established at the urging of Rove and former RNC chairman Ed Gillespie, who wanted to create an alternative campaign outlet for "donors who wanted to give more than the maximum allowed to the RNC," said Collegio.
"It was a plan to combat what the left had done so successfully with MoveOn.org and other groups in the 2008 elections," he said.
American Crossroads' financial filings have offered a glimpse at its donors, who include an array of individuals and businesses whose size of contributions have varied dramatically.
Chief Oil & Gas company CEO Trevor Reese-Jones and billionaire Robert Rowling, whose company, TRT Holdings, owns Omni Hotels and Gold's Gym chains and runs the Grand Ole Opry in Nashville, Tenn., top the list of individual contributors. Each has given $2,000,000.
Cincinnati, Ohio, based American Financial Group, a Fortune 500 company, gave $400,000 -- a contribution that until recently would have been illegal under rules prohibiting direct corporate campaigning for or against particular candidates.
The Supreme Court decisions Citizens United v. Federal Election Commission and SpecechNow.org v. Federal Election Commission opened a "wild new world of money that's difficult to track and be vigilant about who's behind it," said Levinthal.
"These super PACs are springing up so frequently somebody could come out of deep left field and make a splash today," he said.
ABC News' Jennifer Schlesinger contributed to this report.