WASHINGTON -- The super PAC supporting Mitt Romney isn't giving up its relentless assault on Romney's chief primary opponent, Rick Santorum, even after Tuesday night's clean sweep in Wisconsin, Maryland and the District of Columbia. The group, named Restore Our Future, has spent $479,114 across the five states voting on April 24, according to reports filed with the Federal Election Commission.

The spending is overwhelmingly concentrated in the two largest states -- and biggest delegate prizes -- of New York and Pennsylvania. Restore Our Future has dropped $277,844 in New York and $153,448 in Pennsylvania. It should be noted that these states are host to the very expensive media markets of New York City and Philadelphia, and far more money is needed to get on the air there than in the other April 24 states. Moreover, TV ads run in New York City and Philadelphia can reach voters in Connecticut and Delaware, respectively -- and those two states are also voting on April 24.

FEC filings show lower levels of super PAC spending to attack Santorum in the three smaller primary states: Connecticut ($17,314), Delaware ($4,368) and Rhode Island ($26,140).

New York should be like home turf for Romney, not just because the local Republican Party is relatively moderate, but also because his fundraising base of Wall Street bankers and hedge fund investors resides in the New York City area.

Pennsylvania, however, is Santorum's home turf, and a Romney win there would certainly signify the end of the primary season. The pro-Santorum super PAC, with far more limited resources than Restore Our Future has, will likely pump the majority of its money into Pennsylvania to protect its candidate from an embarrassing home loss, something not even the cash-poor Newt Gingrich suffered in his home state of Georgia. The group, named the Red, White & Blue Fund, has yet to report any spending in the April 24 primary states.

So far, Restore Our Future has spent more than $40 million in the Republican primary campaign with nearly all of that money going to attack Santorum and Gingrich. That rate of spending is unprecedented for an independent group in a primary campaign and makes the super PAC the second biggest spender on independent electoral expenditures in recent history, behind only the $42 million spent by the Service Employees International Union during the full 2008 presidential campaign. Restore Our Future will likely move into first place for independent expenditures sometime in April, more than seven months before SEIU set the record in November 2008.

Independent group expenditures have soared since the Supreme Court in its 2010 Citizens United ruling freed corporations and unions to spend freely on elections, so long as they did so independently. The court's finding that money spent by groups independent of campaigns and parties cannot corrupt led to a lower court ruling that freed individuals to pool unlimited money to spend independently on elections. That led to the creation of the super PAC.

According to the Center for Responsive Politics, independent spending in the presidential election as of April 4 is more than double that at the same point in time in the 2008 campaign. Independent groups like Restore Our Future and the Red, White & Blue Fund have combined to spend close to $100 million in the 2012 campaign.

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