washington national rifle association endorse republican donald trump u.s. president friday respond promise serve country top gun lobby group well trump party presumptive nominee nov. presidential election praise nra take swipe democratic front-runner hillary clinton say want abolish u.s. constitution second amendment guarantee right bear arm billionaire businessman oppose gun-free zone promise protect americans right self-defense let trump tell gather louisville kentucky nra country lead gun-rights advocacy group nra top u.s. gun lobby group endorse trump presidential race