President Obama made a surprise visit to his re-election headquarters in Chicago this afternoon before an evening of fundraisers expected to net at least $1.8 million for Democrats and his campaign.

"The people who are working in Chicago and have been for the better part of the last year have been focused on his reelection, have ensured that he's had time to focus on his duties as president of the United States. So this will be an opportunity for him to swing by the office, to say hello to the staff that's been hard at work there and to thank them for their efforts," White House spokesman Josh Earnest told reporters on Air Force One ahead of the stop.

While the press was not allowed into headquarters, the president reportedly spoke briefly to a few hundred members of his staff, reminiscing about his first trip to Chicago as a young man and the early days of his campaign for the state senate. He then shook "many, many hands," Earnest said.

This was Obama's first visit to the sprawling 50,000-square-foot office space at One Prudential Plaza, a skyscraper overlooking Millenium Park, a campaign aide confirmed.  (ABC News recently got an inside look at the operation, which you can learn more about  HERE.)

Obama will spend the remainder of the evening mingling with supporters and donors at three Windy City events, all raising money for the Obama Victory Fund, a joint account to benefit Obama and the Democratic National Committee for 2012.

The president will rally a crowd of more than 500  younger supporters at the University of Illinois-Chicago, where R&B singer Janelle Monae and "CSI: NY" star Hill Harper are featured guests. Tickets started at $44 for Gen 44 members and go up to $100 general admission, a Democratic official said.

He then heads to a glitzy reception at the private residence of media mogul Fred Eychaner, where at least 60 guests will have paid $35,800 per couple for a chance to see the president, the official said.

Eychaner is one of Obama's "bundlers,"  collecting between $50,000 and $100,000 for Obama and Democrats for 2012, according to campaign finance records.

Obama will conclude the evening with a $7,500-and-up per person reception at the home of former Bear Stearns executive Stuart Taylor. More than 100 are expected to attend.

By law, the first $5,000 of an individual's contribution to the Obama Victory Fund goes directly to Obama's campaign, while the rest, up to $30,800, to the DNC.

Obama's last visit to Chicago was on Aug. 3 for a fundraiser and a  50th birthday bash.