President Barack Obama, center, jokes with Sen. Michael Bennet, Colorado Democrat, left, and Colorado Gov. John Hickenlooper after Obama's campaign stop on the campus of the University of Colorado in Boulder, on Sunday, Sept. 2, 2012. (AP Photo/David Zalubowski)

Photo by: David Zalubowski

President Barack Obama, center, jokes with Sen. Michael Bennet, Colorado Democrat, left, and Colorado Gov. John Hickenlooper after Obama's campaign stop on the campus of the University of Colorado in Boulder, on Sunday, Sept. 2, 2012. (AP Photo/David Zalubowski)