Nikki Haley, the former South Carolina governor and United Nations ambassador, five months into her first run for president, acknowledges the position she is in. Though she was the first Republican to announce a challenge to former President Donald J. Trump, she hasn’t spent a dime on television ads, is polling well behind Mr. Trump and Gov. Ron DeSantis of Florida and has struggled at times to make a case for her campaign. But in an interview on Friday, at a picnic table outside a Veterans of Foreign Wars post…