"I've looked him in the eye, discussed this issue, and I absolutely believe in my heart and know that he is personally committed to a pro-life agenda as president," Christie said on the conservative "Laura Ingraham Show."
Christie, too, launched his political career as a supporter of abortion rights.
He has said he changed his mind after attending a prenatal visit with his wife, then pregnant with their second child.
"I heard a heartbeat," Christie told CNN last year. "And it led to me having a real reflection on my position."
Of Romney, Christie said Wednesday that "he stands for a strong pro-life agenda, and that is frankly who he is going to be."
Christie specifically cited Romney's pledges to bar U.S. foreign aid from funding abortions and to stop public health funding for Planned Parenthood.
Democrats, meanwhile, have accused Romney of trying to downplay his abortion views in order to attract female voters as the presidential race tightens.
"He's trying to cynically and dishonestly hide his real positions," said Obama deputy campaign manager Stephanie Cutter on Wednesday, "but there's no hiding when you're president.
“[Romney's abortion stance is] bad for his presidential prospects and now he's trying to cover it up," Cutter said on a call with reporters.