CHICAGO—President Barack Obama stopped by a Democratic victory office here on Tuesday to encourage supporters and call volunteers in Wisconsin. The first person he reached on the phone took some time grasping who was on the other line.

At a downtown Obama for America field office, Obama thanked the staff and volunteers before calling a woman named Annie in Wisconsin.

"Hi is this Annie?," Obama said, according to the pool report. "This is Barack Obama."

There was a brief pause.

"This is Barack Obama," he repeated. "You know, the president?"

When it sunk in that the caller wasn't kidding and was indeed the president, they went on to chat.

"She was very nice to me even though she initially didn't know who I was," Obama joked when he set down the receiver. He went on to make six calls to volunteers. On the final one to Jill in Madison, she didn't seem to believe it at first, either.

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"Hi, is this Jill? This is Barack Obama. This is your president. You're working so hard I had to say thanks," Obama told her. "No, it is. I'm calling some of our best volunteers up in Madison."

When he finished the calls, Obama turned to the crowd at the campaign office and offered a message to his challenger, Republican Mitt Romney.

"I also want to say to Governor Romney, congratulations on a spirited campaign. I know that his supporters are just as engaged and just as enthusiastic and working just as hard today," Obama said. "I'm looking forward to the results. And I expect that we'll have a good night. But no matter what happens, I just want to say how much I appreciate everybody who has supported me, everybody who has worked so hard on my behalf. And again, I want to congratulate Gov. Romney and his team for a hard-fought race as well."

Before he left, according to the pool report, Obama picked up a Starbucks cup, sipped it, and realized it was someone else's."That's coffee, not tea," Obama said. "I thought that was my tea."