PHOENIX, Ariz. — Mitt Romney homed in on chief rival Rick Santorum on Thursday, just hours after the former Pennsylvania senator stumbled in explaining his support for unpopular positions within the conservative movement during a debate here.

During a subpar debate performance in Mesa Wednesday, Santorum defended his backing of No Child Left Behind — an education initiative that significantly broadened the oversight role of the federal government — as part of an effort to support Republican Party leaders.