By Gram Slattery and James Oliphant PORTSMOUTH/KEENE, New Hampshire (Reuters) – Tom Mita, a 45-year-old non-profit worker in Portsmouth, New Hampshire, is not registered with a political party. That makes him the perfect target for Nikki Haley, who needs independent voters for a chance to prevail in this pivotal primary state. Mita is thinking about voting for Haley, he told a pair of door knockers who were canvassing on behalf of the former U.S. ambassador to the United Nations outside his suburban home on Saturday, but he isn’t completely sold on her candidacy. He wants Haley, who has so far…