WASHINGTON -- U.S. Sen. Bill Nelson is facing a growing threat in his re-election campaign — and it’s not necessarily Republican rival U.S. Rep. Connie Mack.

It’s the super PAC.

American Crossroads, a political action committee overseen by conservative strategist Karl Rove, said this week it has reserved $6.2 million in air time this fall to link Nelson to President Barack Obama’s agenda.

Nelson "has a long record that makes him vulnerable," said Crossroads spokesman Nate Hodson.

That follows a $1 million injection into a new super PAC supporting Mack. The money to Freedom PAC came from casino billionaire Sheldon Adelson, who has already donated millions to super PACs supporting Newt Gingrich and Mitt Romney.

And in Tampa this week, anti-Nelson TV ads began running from another newly formed group, American Commitment, a tax exempt advocacy group that is like a super PAC but does not have to disclose its donors.

The ad attacks Nelson for supporting the health care law and is steeped with distortions but underscores how outside groups have dramatically changed the way campaigns are waged. Republicans already control the House — thanks in part to a massive advantage in outside spending in 2010 — and need to win four net seats to take the Senate. Other Democrats are considered more vulnerable than Nelson, but the Republicans are spreading their odds.

Nelson added $1.8 million to his campaign coffers in the quarter that ended June 30 and now has nearly $11 million cash on hand, a formidable amount and far more than Mack, whom some Republicans worry is not strong enough to defeat the two-term Nelson.

Mack’s campaign is way behind. It reported Wednesday raising about $839,000 for the quarter and has $1.3 million cash on hand. But the third-party money levels the advantage and ensures Nelson will be pummeled with ads leading up to the Nov. 6 election. Polling already suggests he’s been hurt.

Other groups, including Americans for Prosperity, 60 Plus Association and the U.S. Chamber of Commerce have already financed ads against Nelson. The ads mostly focus on Nelson’s support of Obama’s major policies, including the stimulus and health care.

All told, more than $14 million has been spent or reserved in air time, including the new American Crossroads $6.2 million, according to media tracking figures confirmed by Nelson’s campaign.

Crossroads has emerged as one of the most aggressive super PACs since it began to proliferate after a 2010 U.S. Supreme Court decision ended restrictions on corporate and union spending in elections.

Along with a sister group Crossroads GPS, which is organized as an educational group under section 501(c)(4) of the tax code and is not required to disclose donors, Crossroads plans to spend $300 million on elections this year.

On top of the money against Nelson, the groups are on track to spend more than $24 million in Florida to oppose Obama or his policies. To put in perspective, that’s more than the $20 million Charlie Crist raised for his 2006 gubernatorial election.

Money has come from corporate interests and wealthy donors such as David and Charles Koch, billionaire industrialists who fund Americans for Prosperity. An AFP ad against Nelson in June cast him as a big spending liberal with failed results.

Adelson, the casino magnate, is another major donor to super PACs and almost single handily kept Gingrich competitive in the Republican presidential primaries, leveling some of the hardest-hitting attacks on Romney’s business record.