The poll, released Saturday evening, showed Romney with 35 percent to Santorum's 31 percent, within the 4-percent margin of error. Newt Gingrich had 12 percent and Ron Paul had 7 percent, with 16 percent undecided.
The poll found, however, that 46 percent of those surveyed said they could change their mind before the March 20 primary.
As in other states, Romney leads among more moderate suburban GOP voters -- he has a 39 percent-to-30 percent edge in Cook County. But Santorum has the lead, 35 percent to 29 percent, among the 96 counties outside the Chicago area.