Former Pennsylvania senator Rick Santorum, seen here speaking at a rally in Oklahoma City, Feb. 9, 2012, has emerged as Mitt Romney’s leading challenger for the GOP presidential nomination.

Former Pennsylvania senator Rick Santorum, seen here speaking at a rally in Oklahoma City, Feb. 9, 2012, has emerged as Mitt Romney’s leading challenger for the GOP presidential nomination.

Former Pennsylvania senator Rick Santorum, seen here speaking at a rally in Oklahoma City, Feb. 9, 2012, has emerged as Mitt Romney’s leading challenger for the GOP presidential nomination.