"See you on the 15th," Nikki Haley says in a tweet pinned to the top of her Twitter page. That’s when the former South Carolina governor who served as ambassador to the United Nations during former President Donald Trump’s administration is expected to declare her candidacy for president, joining Trump as the only major Republicans to date to have launched White House campaigns. Since Trump jumped into the presidential race in mid-November, it’s been a field of one in the hunt for the GOP nomination. But Haley’s entry — at an event in Charleston, South Carolina — could open the...