WASHINGTON (Reuters) - Rick Santorum held a slight lead over Mitt Romney in the Republican presidential primary in Ohio with a quarter of the vote counted on Tuesday.

Santorum was leading with 39 percent to Romney's 36 percent with 25 percent of the vote counted in Ohio, the most closely watched race of the 10 nominating contests being held on Super Tuesday.

(Reporting by Deborah Charles; Editing by Doina Chiacu)