Reporting from Florence, S.C.— Newt Gingrich on Tuesday questioned why Mitt Romney has not released his tax returns, arguing that if the front-runner for the GOP presidential nomination has nothing to hide, he ought to make the documents public before South Carolina holds its first-in-the-South primary on Saturday. "Either there's nothing there, so why isn't he releasing them, or there's something there, so why is he hiding them?" Gingrich told reporters here after holding a town hall at an art gallery. "I believe we have the right to know. Therefore, he owes us the knowledge. If there's nothing there, why...