Each candidate has 49 percent of the vote, results indicated. The margin of error is 4.5 percent.
Wisconsin is the home state of Romney's running mate, U.S. Rep. Paul Ryan.
Rasmussen rated the race in Wisconsin a tossup in its Electoral College projections.
Obama carried Wisconsin by a 56 percent-to-42 percent margin in 2008.
A week ago, Rasmussen said its poll indicated Obama held a slight 50 percent-to-48 percent edge over Romney.
The survey of 500 likely voters in Wisconsin was Thursday.