“A lot of people, when Trump first started talking about it, thought this is crazy,” said John Yoo, a law professor at the University of California, Berkeley, School of Law, who was a top lawyer in the George W. Bush administration. “But in the intervening years, a lot more serious people are taking it seriously.” Even as the legal debate has grown more robust, many legal experts, including Professor Yoo, remain confident that a majority of justices across the ideological spectrum will rule against Mr. Trump’s quest to redefine citizenship.