That was a reference to a commercial aired by Priorities USA Action, which supports Obama, in which a retired steel worker suggests Romney and his private equity firm, Bain Capital, bear some responsibility for his wife's death from cancer.
The ad has been judged harshly by independent fact checkers and vociferously disputed by Romney's campaign. Even so, Obama's re-election aides decline to disavow it, noting they are barred by law from coordinating efforts with the outside group.
Campaigning in person, the rival tickets ranged over some of the nation's most heavily contested terrain.
Obama set out on a three-day tour of Iowa, traveling in a black bus bearing the presidential seal on the side. Ryan was in Iowa, too, Biden in North Carolina, Romney in Florida.
Ryan did not mention Medicare as he made his debut as a solo campaigner on the Republican ticket. Instead, in an appearance at the Iowa State Fair outside Des Moines, he said Obama "is spending our children into a diminished future."
Noting that Obama was also in the state, he told his audience, "As you see the president come through in his bus tour, you might ask him the same question that I'm getting asked from people all around America. And that is, 'Where are the jobs, Mr. President?'"
Fairgoers who attended his brief address were largely positive. Protesters interrupted him several times, chanting "Stop the war on the middle class," and one woman climbed on stage with Ryan before security could drag her away.
"She must not be from Iowa," Ryan said.
Obama had a jab of his own as he spoke to a crowd in Council Bluffs in the dusty, drought-ridden Midwest.
He said Ryan was among House Republican leaders blocking passage of legislation that "not only helps farmers and ranchers respond to natural disasters but also makes some necessary reforms and gives farmers and ranchers some long-term certainty ...
"So if you happen to see congressman Ryan, tell him how important this farm bill is to Iowa and our rural communities."
Romney's campaign dismissed the criticism. "No one will work harder to defend farmers and ranchers than the Romney-Ryan ticket," said spokesman Ryan Williams.
In criticizing Ryan and other Republican lawmakers, Obama omitted that the GOP-controlled House approved a short-term drought relief bill before Congress left the Capitol for a long summer break. The measure is stuck in the Democratic-controlled Senate, where leaders prefer action on a longer-term farm bill that cleared on a bipartisan vote and includes the emergency aid.
With lawmakers gridlocked and away from the Capitol, Obama said the government would buy more than $150 million worth of meat and fish to help producers.
Later Monday, and a few hours after Ryan's appearance, Obama made an hour-long surprise stop at the Iowa State Fair. He greeted throngs of parents and their children, and later enjoyed a Bud Light and a pork chop with Agriculture Secretary Tom Vilsack, a former governor of the state.
A large crowd outside the Bud Tent chanted "four more years," and then some yelled, "Four more beers!"
Holding a beer in a clear plastic cup, Obama said, "It's going to go well with my pork chop."
Kasie Hunt reported from Florida. Associated Press writers Matthew Daly in North Carolina, Steve Peoples in Iowa and David Espo in Washington contributed.
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