President Barack Obama’s re-election campaign strategist David Axelrod warned today that the president’s campaign would respond aggressively to the “Karl and Koch brothers’ contract killers in super PAC-land” and treat their ads as if they came from Mitt Romney himself.

Axelrod was referring to GOP strategist Karl Rove’s group American Crossroads and the billionaire Koch brothers, Charles and David, who have bankrolled several conservative political action committees. Both are expected to play a large role in attacking Obama and other Democratic candidates this year.

On a conference call with reporters during which the campaign detailed a $25 million positive ad buy in swing states during the next month, Axelrod said Romney can’t sell his policy prescriptions, which the strategist ripped as backward-looking and as a doubling down on tax cuts for the wealthy.

“He’s basically reduced to running a negative campaign, just as he was in the primaries,” Axelrod said. “We have a different approach.”

The first new ad debuted Monday, pointing to the crisis when Obama began his presidency and touching some of the highlights of his first term — although it does not mention his signature health care reform law.

Axelrod said that while the Obama campaign would focus on a positive message aimed at the future, it would return fire from the Romney campaign and its allies.

“We’re going to be prepared, and I want to be clear, to respond to these attacks,” Axelrod said, including attacks from the “Karl and Koch brothers’ contract killers in super PAC-land.”

“We will treat every ad that comes from those entities as a Romney ad,” he said.

Axelrod and campaign manager Jim Messina contended that Democratic enthusiasm is up and will continue to build into the fall with a positive approach. Romney, however, “invested tens and tens of millions of dollars simply ripping other people down” in the primaries, Axelrod said.

“We believe this election is about the future and how you build a butter future for the middle class. That’s going generate more enthusiasm than this grinding negative campaign.”

Axelrod also said he believes the large gender gap showing that women prefer the president will persist.

“Gov. Romney would take us back 50 years” on issues such as contraception, Axelrod said. “I don’t think that these numbers are accidental, and I don’t think they’re reversible.”

The Romney campaign responded quickly to the call via email.

“President Obama would like for voters to believe he hasn’t been president for the last three years,” said Amanda Henneberg, Romney campaign spokeswoman. “It’s harder to get a job, buy or sell a home, and those fortunate enough to have jobs often have less in their paychecks. Mitt Romney will get our country back on track and stop the middle-class squeeze of the Obama economy.”

The Obama ad buy will start with the ad “Go,” which will air in Virginia, Pennsylvania, Ohio, Nevada, New Hampshire, Iowa, North Carolina, Florida and Colorado, according to the campaign.

Axelrod also repeated his contention that Vice President Joseph Biden’s remarks Sunday saying he was “comfortable” with gay marriage did not mean there was a split with the president, who has said he is “evolving” on the issue.

The president believes that couples who are married, whether gay or heterosexual “are entitled to the very same rights,” Axelrod said, noting that Obama believes the Defense of Marriage Act is unconstitutional. “When people are married, we ought to recognize those marriages,” he said, adding that “there couldn’t be a starker contrast” with Romney, who supports a constitutional amendment banning gay marriage.