Against Sanders, the facts of his open support for Communists in the Caribbean and Central America into the Reagan era, as well as his apparent support for the Iranian Revolution, would be dinned in to the public. Sanders is often praised by his followers for 'consistency' throughout his political career, and this is where he was at its beginning. These are not smears, these are positions of a party he stood as elector for in a Presidential election. It is hard to claim a man does not support positions of a candidate he is an elector for. It can be attempted, but not with much hope of success. 'Crazy Bernie the Commie' is simply too fat a target to be passed up, and it would not be.
The 'Crazy Bernie the Commie' line will bite home. There is just enough fact, and fact not too widely known, to give it real bite. Especially among older voters, who were of age during the Cold War. And the line would be helped along by Sanders inability not to say that one extra sentence to try and justify his youthful foolishness. Of course he wasn't really youthful, he was nearing forty years of age. No one really cares if 'Red China' reduced poverty, no one really thinks that quite excuses the Great Leap Forward or the Cultural Revolution or Tiananmen Square. Just as no one, particular people of Cuban heritage in Florida, gives the north end of a southbound rat whether Castro got more people able to read.
There does not seem to me any obvious way Sanders defends himself against this line of attack, pressed home in news coverage of rallies (video or otherwise) and the steady bombardment of attack ads on the airwaves, and of crafted personal communications on social media. This line of attack is something no Democrat wants to press against Sanders, out of concern for the sensibilities and loyalties of his supporters and followers. Our open enemies would have no such compunction.
"From Bernies perspective, dropping out of a race once you have no chance of winning is peculiar behavior that can only be explained by the work of a hidden hand. For most politicians, though, it is actually standard operating procedure. Only Sanders seems to think the normal thing to do once voters have made clear they dont want to nominate you is to continue campaigning anyway."
"When things are not called by their right names, what is said cannot make sense. When what is said does not make sense, what is planned cannot succeed. When plans do not succeed, people become uneasy. When people are uneasy, punishments do not fit crimes. When punishments do not fit crimes, people cannot know where to put hand or foot."