The Obama campaign began the week by announcing a whopping $25 million ad buy in nine battleground states — and their top strategists told reporters that it is only the beginning.

President Obama speaks at a campaign rally at the Virginia Commonwealth University May 5, 2012 in Richmond, Va. (Sara D. Davis/GETTY IMAGES)

“We’re not letting our foot off the pedal,” said campaign manager Jim Messina in a conference call with reporters Monday morning. “We have a very simple choice between going forward and going back.”

Messina, along with senior strategist David Axelrod, told reporters that the ad, called “Go,” is part of a larger campaign effort to convey all that President Obama has done since taking office in the midst of an economic crisis and two wars in January 2009.

Axelrod said the president’s decision to talk positively about his record on the auto-industry bailout, winding down the wars in Iraq and Afghanistan, going after al-Qaeda and other achievements contrasts starkly with the strategy of his Republican opponent, Mitt Romney, to blame the nation’s continuing economic difficulties on the current administration without offering credible solutions.

“We’re in a different place,” Axelrod said. “We’re fighting our way through. There are still significant headwinds.”

Axelrod didn't preclude the need to respond to Romney’s attacks — and those of independent Republican super PACs, which he said the Obama campaign will respond “vigorously” to and treat “as an ad from governor Romney.”

Axelrod went so far as to describe the conservative Koch brothers, who have bankrolled several anti-Obama efforts including the free-market group Americans for Prosperity, as “contract killers over there in super PAC land who are going to continue to pound away on behalf of governor Romney.”

Axelrod didn’t mention the negative ads that Obama has aired already, including one broadcast in Ohio, Virginia and Iowa this month slamming Romney for holding a Swiss bank account.

A Romney campaign spokeswoman, Amanda Henneberg, did not respond to an inquiry about whether Romney would answer the Obama ad buy with one of his own, but she offered this rebuttal to the Messina/Axelrod call:

“President Obama would like for voters to believe he hasn’t been president for the last three years. Americans are disappointed in President Obama’s liberal policies that haven’t made their lives any better. President Obama just hasn’t lived up to his promises. 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.”

Axelrod and Messina were also asked about Vice President Biden’s remarks on gay marriage during an appearance on “Meet the Press”Sunday, which have been widely interpreted to reflect a more pro-gay-marriage stand than the administration has adopted to date.

Biden said he is “absolutely comfortable” with gay marriage. On the call Monday, Axelrod said the vice president’s remarks are entirely in sync with the administration’s.

Axelrod said the issue offers yet another contrast between Obama and Romney, and that Romney’s support of several anti-gay marriage initiatives and for a constitutional amendment banning the practice are backward-looking positions that most Americans reject.

“There’s a very clear distinction in this race,” he said.

During the call, Axelrod also discussed Vice President Biden’s remarks Sunday that he supports legalizing same-sex marriage.