"I just don't think the president and his people understand that as they burden enterprise with taxation and with regulation, they hurt all of us," Romney said.

Romney's comments came hours after chief rival Rick Santorum appeared outside the Supreme Court to protest the legislation. During his comments, Santorum said Romney — who supported legislation similar to the president's on the state level while governor of Massachusetts — was "uniquely disqualified to make the case" against healthcare reform.

“The worst person to make that case is Mitt Romney and that's why I say we're here today and he's not," Santorum said.

Monday was the first of three days of scheduled oral arguments before the court, with justices weighing whether or not they could rule on the legislation's individual mandate before the legislation goes into effect, sometime after 2014.