WASHINGTON, Feb 15 (Reuters) - President Donald Trump said on Friday he would declare a national emergency at the U.S.-Mexico border, a move expected to plunge him into a fight with Democrats over what they call an unconstitutional attempt to fund a wall without approval from Congress.Trump had demanded Congress include money for the wall, one of his biggest 2016 campaign promises, in a funding bill he was expected to sign either later on Friday or Saturday.