Republican front-runner Donald Trump said on Wednesday that when it comes to the polls in the crowded 2016 presidential race, "the only thing constant is Trump."Trump, a real estate mogul who has generated support and controversy with provocative comments on illegal immigration, has rocketed to the top of the polls among a field of 17 Republican presidential candidates.Asked at a news conference ahead of a town-hall forum in New Hampshire who he saw as his strongest rival in the crowded Republican field, Trump said: "You know it changes. The only thing constant is Trump.""They're going up and down like yo-yos," he said of his Republican rivals. "But I've been up there for a long time and I hope I'm going to be up there for a long time."Saying he had seen newspaper reports that he would grow tired of the presidential race, Trump said: "I'm not going anywhere folks. I'm not doing this for my health. I'm doing this to make America great again."