At one of his five Chicago fundraisers today, President Obama made his first remarks about his budgetary arch-nemesis, Rep. Paul Ryan (R-Wis.), entering the presidential race as Mitt Romney’s No. 2.

One of the events — and the most expensive, at $40,000 a head — will be at Obama’s Hyde Park home, where he’s staying for two nights.

The Gen44 fundraising reception was billed as an event with 1,000 people at the Bridgeport Arts Center, but The New York Times’ Jodi Kantor tweeted: “At Obama fundraiser in Chicago. Admission only $51, but room is half full.”

As soon as Obama mentioned that Ryan, “the ideological leader of the Republicans in Congress,” had gotten the nod from Romney, the audience began booing.

“No, no, no, no. Look, I want to congratulate Congressman Ryan. I know him. I welcome him to the race,” the president said. “Congressman Ryan is a decent man. He is a family man. He’s an articulate spokesman for Governor Romney’s vision. But it’s a vision that I fundamentally disagree with.”

Obama and Ryan have long had public fundamental disagreements, most notably tangling at the 2010 heathcare summit.

Obama did not mention Ryan further in his speech, nor did he mention Medicare.

After leaving Chicago tomorrow, the president flies to Omaha, Neb., to begin a three-day campaign bus tour through Iowa. Ryan’s campaign stops in swing territory include central Florida next weekend, where he’ll tackle Democratic accusations that his Medicare plan would leave seniors high and dry.