WASHINGTON—Mitt Romney has easily won the Illinois Republican primary over rival Rick Santorum.

With half of the precincts reporting, Romney had 50 percent and Santorum 33 percent. Ron Paul and Newt Gingrich, who did not campaign heavily in Illinois, were far behind, with 9 percent and 8 percent respectively.

In Illinois, as in Michigan and Ohio, Romney enjoyed an enormous advantage in television advertising. His campaign and Restore Our Future, a super PAC that supports him, outspent Santorum and his super PAC by $3.5 million to $500,000, an advantage of 7-1.