"My little nephew came to me and he said, 'Uncle, what's the difference between a hypothetical question and a realistic question?'" Mac said. "I said, I don't know, but I said, 'Go upstairs and ask your mother if she'd make love to the mailman for $50,000.'"
As the joke continued, the punchline evoked an angry response from at least one person in the audience, who said it was offensive to women.
"It's not funny. Let's get Barack on," a man shouted from the crowd, which paid $2,300 each to support the Illinois senator.
About 15 minutes later, Obama tried to smooth things over with a joke of his own.
"We can't afford to be divided by race. We can't afford to be divided by region or by class and we can't afford to be divided by gender, which by the way, that means, Bernie, you've got to clean up your act next time," Obama said. "This is a family affair. By the way, I'm just messing with you, man."
The incident drew response from Obama's campaign, which criticized Mac for his choice of material.
"Sen. Obama told Bernie Mac that he doesn't condone these statements and believes what was said was inappropriate," spokeswoman Jen Psaki said in a statement after the event.
Mac, a Chicago native, said he's a longtime Obama supporter and called the presumptive Democratic nominee a "man's man" while offering him advice for the duration of the campaign trail.
"People like rumors. They're going to say things like you was at the club with Lil' Kim, and you and Kanye West got into a fist fight," he said. "You can't get upset. You've got to keep hope alive."
Mac's appearance was Obama's first celebrity event of the evening. Later at a Lincoln Park nightclub, Obama spoke to a raucous crowd of music fans, who paid up to $500 per person to see a performance by Wilco lead singer Jeff Tweedy, and two other band members.
"Before these guys go, I want them to know that I had heard a rumor that they had suggested that I had nothing by them on my iPod," Obama said. "That is not true. I love Wilco."
