Officials said no one was wounded in the attack whose motive and target was being investigated.
Denver police spokeswoman Raquel Lopez said Staff were in the office when the incident occurred mid-afternoon.
“”It looks like it was one shot that was fired into the structure,” she said.
A photograph posted online by the Denver Westword showed a window broken at the front of the office.
Obama campaign officials did not immediately comment about the incident, which comes as Obama and Republican rival Mitt Romney battle into the final weeks before the November 6 presidential election.