The Chicago Tribune reported the Democratic incumbent was favored by 49 percent while the Republican challenger is the pick of 48 percent.
A recently as two weeks ago, the Marquette poll had Obama leading 53 percent to 42 percent.
The poll of 870 likely voters was conducted Oct. 11-14, before the second presidential debate, and has a 3.4-percentage point margin of error.
Romney made inroads with independents since the first debate between the two presidential contenders. He has gone from a 9-point deficit among independents to a 4-point advantage, the Tribune said.
Obama also has lost traction among Wisconsin women, with his advantage slipping from 61-37 to 51-47.