The advertising push includes large ad buys in multiple markets in Florida, Ohio, North Carolina and Colorado, and will cover a number of 30-second and 60-second spots running into early June.
In a conference call with reporters, David Axelrod, a senior adviser to the Obama campaign, said the campaign would devote its May advertising to a positive message touting Obama's accomplishments but was prepared to respond to criticism from "the Karl and Koch brothers' contract killers over there in super PAC land."
It was a reference to outside groups linked to Karl Rove, a former adviser to President George W. Bush, and the heads of Koch Industries, longtime supporters of conservative causes. Obama has responded to a handful of critical ads in recent months with his own defensive spots, and Axelrod criticized the GOP-leaning groups even though the White House has signaled to Democratic donors that they too should donate to Democratic-leaning super PACs.
While Obama let his campaign ad shape the race Monday, Romney headed to suburban Cleveland. He softened his tone at times, sharing the stories of struggling Americans he's met on the campaign trail and countering the notion that he came from a privileged background. He said his father, a former auto executive, never had the time or money to get a college degree and his parents "couldn't afford a fancy honeymoon" when they married.
During Romney's town hall meeting, a woman said in a question to Romney that Obama had strayed from the principles of the Constitution and "should be tried for treason." Romney did not respond to her suggestion of treason but told reporters later that "no, of course" the president should not be tried for such an offense.
On Tuesday he was heading to Michigan, the state where he grew up and has identified as a potential Republican pickup, and on Wednesday to Colorado, where Obama staged the 2008 Democratic National Convention and captured electoral votes a few months later. He was to visit the state capitals of Lansing and Denver.
Romney, the presumptive GOP nominee, was expected to add more delegates to his haul from Tuesday's primaries in North Carolina, Indiana and West Virginia. He has 856 delegates, according to The Associated Press' count, nearly 300 delegates short of the 1,144 needed to win the nomination.
His one-time opponent Rick Santorum, whose departure from the primary battle last month confirmed anew that Romney had the nomination all but in hand, endorsed Romney in a late-night email to supporters.
"The primary campaign certainly made it clear that Gov. Romney and I have some differences. But there are many significant areas in which we agree," Santorum wrote, citing common ground in economic, social and foreign policy.
"Above all else, we both agree that President Obama must be defeated," he said. "The task will not be easy. It will require all hands on deck if our nominee is to be victorious. Gov. Romney will be that nominee and he has my endorsement and support to win this the most critical election of our lifetime."
Obama, for his part, was taking his economic message to events in Albany, N.Y., on Tuesday, and Reno, Nev., on Friday. He also was holding fundraisers later in the week in Seattle and Los Angeles, where he was attending a high-dollar dinner at the home of actor George Clooney.
Thomas reported from Washington. Associated Press writers Julie Pace, Stephen Ohlemacher, Jack Gillum and Philip Elliott in Washington contributed to this report.