Brynn Anderson/APHUNTSVILLE, Alabama (Reuters) - President Donald Trump injected himself into a bitter U.S. Senate primary fight in Alabama on Friday, putting to the test his ability to enlist his anti-establishment voters to come to the aid of an endangered Republican incumbent. Trump spoke at a rally in Huntsville, Alabama, on behalf of Senator Luther Strange, who was appointed after the seat was left vacant when Jeff Sessions was named Trump's attorney general. Strange is trying to ward off a challenge from Roy Moore, an arch-conservative former state Supreme Court justice, in a runoff election next week.