MILWAUKEE, WI - NOVEMBER 03: Pop artist Katy Perry performs at a campaign rally for U.S. President Barack Obama at the Delta Center on November 3, 2012 in Milwaukee, Wisconsin. Wisconsin is one of nine battleground states expected to determine the outcome of Tuesday's U.S. Presidential election. (Photo by Scott Olson/Getty Images)
Perry came on stage at Saturday's event initially wearing a red, white and blue dress and holding a microphone shaped like the Statue of Liberty's torch.
But after her first song, a cover of Al Green's soul hit "Let's Stay Together," she tossed aside the dress to reveal the bright blue minidress.
Obama's slogan "Forward" is also Wisconsin's state motto.
Perry paused midway through her set to make a pitch for donations to victims of Superstorm Sandy.
Obama's rally with Perry comes before he's set to be in Madison on Monday with rocker Bruce Springsteen.
(Copyright 2012 by The Associated Press. All Rights Reserved.)