Conservative super-PACs are âcontract killers,' according to Obama campaign manager President Barack Obama's re-election campaign on Monday denounced Mitt Romney's super-PAC allies as "contract killers" as it unveiled a $25 million advertising blitz to shore up the Democrat's re-election prospects in key battleground states. "We're not letting our foot off the pedal," campaign manager Jim Messina told reporters on a conference call. The first adÂ focuses on the economy. We're also going to be prepared, and I want to be clear, to respond to the attacks that we expect to continue from, not just from the Romney campaign, but from, you know, the Karl (Rove) and Koch brothers contract killers over there in super PAC land, who are going to continue to pound away on behalf of Gov. Romney, "We will respond vigorously, we will treat every ad that comes from those entities as an ad from Gov. Romney and we will compare our record and our vision with his, and we'll let the American people decide," he added. The Obama campaign has hardly been shy about going negative against Romney: Recent ads have suggested that the former Massachusetts governor would not have ordered the raid that killed Osama bin Laden and made political hay from Romney having a Swiss bank account. And the president himself assailed his rival in sharply personal terms at two rallies over the weekend, accusing him at one point of having proposed "cutting his own taxes while raising them on 18 million working families." The Romney campaign "likes to pretend that world history started in 2009," scoffed Messina, who said that Obama has faced "a combination of crises few others have had to deal with." David Axelrod, an Obama strategist, charged that the Romney campaign and super PACs supporting him have spent roughly $55 million in the campaign so far, and "close to 90 percent of it has been attacking opponents." "I believe that by the end of this week, certainly by the end of next, we'll have spent more money offering people a positive vision for the future" than Romney has "in his entire campaign."