Three decades ago, Loudon Wainwright III paused during his First Avenue set, swayed precipitously, and contemplated the impending election of George H.W. Bush. “Oh, no, that asshole is gonna win,” he moaned. Now faced with an even more appalling Republican candidate, Wainwright is back with “I Had a Dream,” copping an MLK line to imagine the nightmare of a Trump presidency. In a revolting and hilarious video made with Judd Apatow, Wainwright spits up his “Trumpweiser” beer after Trump, “his little finger on the button … blew up Cuba, he carpet-bombed Montreal,” named Rush Limbaugh secretary of hate, and put Lyin’ Ted Cruz on the Supreme Court. Live, it would seem apropos to follow up this new gem by stinkin’ it up with his lone, ancient hit, “Dead Skunk.” LWIII has cleverly chronicled myriad personal and societal foibles for years with lacerating sarcasm, leaving no one — especially himself — unscathed. His latest album, 2014’s Haven’t Got the Blues (Yet), tackles meth, guns, parking spaces, and the rockabillyin’ creaks and odors of his own elderly self.