A pivotal justice sent mixed signals as the U.S. Supreme Court divided along ideological lines during arguments in a historic case that could legalize gay weddings nationwide.

Justice Anthony Kennedy said same-sex couples were seeking the same "dignity" and "ennoblement" as heterosexual couples. Yet earlier, he offered a glimmer of hope to people who say marriage should be defined as between a man and a woman.

"This definition has been with us for millennia," Kennedy said. "And it's very difficult for the Supreme Court to say, 'Oh, well, we know better.'"