Rick Santorum and Mitt Romney are fighting to break into a lead in Ohio.
With 79 percent of the vote counted, the two were less than a percentage point apart at 38 percent for Romney and 37 percent for Santorum.
Each man has won a trio of other states, with Romney triumphing in Massachusetts, Virginia and Vermont. Santorum has won North Dakota, Tennessee and Oklahoma.
Earlier, Newt Gingrich won the GOP contest in his home state of Georgia.
A total of 10 states are voting on Super Tuesday.