Why do Democrats insult or make accusations against opponents rather than discussing or justifying the issues?

They do it to distract from the issue. See, they do this for several reason. One being they have no argument to stand on so they try to surround their opponent in an irrelevant issue. Two they have a very week argument that holds no wait against their opponent's argument. Three, they hold a stance on the issue that would be very unpopular so the skirt the issue by slinging mud. Four, they don't want to give an answer because they don't want to be committed to taking any action if elected.