CHICAGO — President Obama addressed his rival’s vice-presidential choice for the first time on Sunday, calling Representative Paul D. Ryan of Wisconsin a nice guy whose leadership would be a national disaster.

“Congressman Ryan is a decent man, he is a family man,” the president said, quieting boos from the audience, but also a spokesman “for a vision I fundamentally disagree with.”

Speaking to hundreds of young supporters at the second of five fund-raising events he is holding here Sunday, Mr. Obama wove Mr. Ryan seamlessly into the speech he has been making for months. In a sense, Mr. Ryan has made Mr. Obama’s rhetorical job easy: for months, the president has argued that the election is a choice between two different paths for the country, and that the Romney/Ryan path of aggressive budget cutting and deregulation will lead to yet more economic suffering. So he dropped just a few words about Mr. Ryan into his address, describing the anxiety and insecurity he said Mr. Ryan’s proposed policies would cause.

“It’s not speculation, it’s on their Web sites,” he said. “They have tried this before, they have tried to sell us this trickle down fairy dust before.”

Through an accident of timing, Mr. Obama finds himself back in the places that created his career — Chicago and Iowa — just as the fight to defend his presidency takes on new definition. Sunday’s events provided a miniature snapshot of the president’s fortunes at the moment. On the one hand, his central argument — that the election is a choice between two visions for the country — received a shot of rhetorical energy from Mitt Romney’s selection of Mr. Ryan, who has argued for changes to Medicaid and Medicare as a way of cutting the deficit.



On the other hand, Mr. Obama is trailing in the money, and his Chicago “birthday” trip is a frenetic dash for cash, from the large, low-dollar event at the arts center to a party in his own backyard with a $40,000 admissions fee.

The Obama campaign does not release fund-raising tallies for individual events, but a simple series of numbers tells the story of their efforts to raise cash. The president and the first lady are dividing and conquering, with Mr. Obama in Chicago and the first lady covering the Rocky Mountains and California over the course of several days. Admission to the arts center was low, with some supporters admitted for as little as $51, a bargain price to see a sitting president up close. Though the Obama campaign declared the event sold out, the loftlike space looked about half full. (A campaign official later said that a thousand donors attended, beyond initial expectations.) Meanwhile, Mark Knoller of CBS, an unofficial archivist of the presidency, said he could not remember another occasion in which a sitting United States president held a fund-raiser in his own home.

The arts center event also provided an unplanned contrast with Saturday’s scene at Mr. Romney’s announcement. That took place on the decommissioned U.S.S. Wisconsin in Virginia, in front of what looked like a mostly white crowd. The Bridgeport Art Center sits on a barren block of Chicago’s South Side, and Mr. Obama spoke front of a diverse, young audience.

Just outside, another reminder of the politics of Chicago’s South Side was visible on the sidewalk. Members of Chicago’s Occupy Wall Street group covered the pavement with chalked statements of protest: “Unhappy 51 to Obama,” the pastel message said. “Stop killing brown people with your drones.”