The comparison with Mr. Corbyn and Labour overseas makes it plain. Mr. Corbyn had said and done things, over a career as a leftist gadfly on the back benches, which made it easy to suggest he was no patriot, that he would side with the other party in a conflict or dispute involving Great Britain. To be perceived as lacking in patriotism, let alone as being anti-patriot, is fatal in a democratic polity. A great preponderance of people value patriotism highly, to the point many are actively repelled by persons they perceive have not got it.
Sanders by attacks on this line can readily be made, and would readily made, to appear to not just lack patriotism but to actually be an anti-patriot. Someone who supported the other side in a global conflict, during which the very existence of the nation was at stake. Many of us on the left feel we are a bit above such things, Sanders certainly does, and this makes defense against the line almost impossible, because the person subject to the attack is genuinely bewildered by it, and the instinct to explain, to educate, even to preach, perhaps the most natural reactions when a person is confronted with something he or she perceives rooted in ignorance, will prove fatal if indulged in. It will simply accentuate the person's evident difference from the common national sentiment, and his feeling he is above the rest, and feels he is better than they are. That leads to electoral oblivion.
"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."