According to
this AP story, apparently the guy made a statement to the effect that he believed Sen. Ernst is a member of the Islamic State, and threatened to kill her at a particular appearance she was about to make.
Most articles "out there" have almost no info about the guy, but based on that one, he sounds like a loony-toon, a burnt-out druggee, or a member of some conspiracy-theory driven paranoid anri-government group. That latter option be pretty much hypothetical, I'll go with paranoid-schizophrenic loony-toon for now. Not that that precludes him also being a DU-member.