A new and powerful political action committee tied to Republican strategist Karl Rove, fueled by out-of-state corporations and billionaires, is swooping into Florida to put its money behind front-running U.S. Senate candidate Marco Rubio.

American Crossroads has spent nearly $250,000 so far on pro-Rubio mailings aimed at absentee voters, according to a report filed this week with the Federal Elections Commission.

The group can shell out unlimited money on Rubio's behalf, thanks to a controversial Supreme Court ruling earlier this year that allows corporations and wealthy individuals to funnel money directly to campaigns.

"This group is really at the forefront of this new phenomenon of Super PACS," said Dave Levinthal, a spokesman for the Washington-based Center for Responsive Politics, a campaign watchdog group. "We've been saying that the Supreme Court decision would open up a brand new avenue of money and influence, and we're seeing that very much come true."

American Crossroads is among a number of outside, special interest groups influencing the race for Florida's coveted open Senate seat and helping Rubio erode the fundraising edge held by Gov. Charlie Crist. The pro-business U.S. Chamber of Commerce and the anti-tax Club for Growth have already aired ads attacking Crist. The National Republican Senatorial Committee has pledged to spend $2.5 million for Rubio.

As of early August, the Republican-turned-independent governor had $8 million stashed away, compared to Rubio with about $4.5 million and Democrat Kendrick Meek with about $2.6 million.

"It's a great thing that Karl Rove and others have come to support Marco's campaign, and I think it's going to seal the deal in getting his message out," said Miami Republican fundraiser Jorge Arrizurieta.

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