Mitt Romney won the Illinois Republican party presidential primary on Tuesday, tightening his grip on the nomination.

Romney has won 47 percent of the vote, easily beating his main challenger, Rick Santorum, who had 35 percent.

Ron Paul won just 9 percent and Newt Gingrich 8 percent, after choosing not to campaign in the state.

Romney's aides hope that winning Illinois will give him an unassailable lead in the race for the party nomination, after he lost to Santorum in conservative southern states, U.S. media report.