“A fifth column is any group of people who undermine a larger group—such as a nation or a besieged city—from within, usually in favor of an enemy group or nation. The activities of a fifth column can be overt or clandestine.”
In this case it’s a political party.
Bernie Sanders has been an independent who caucuses with the Democratic party. When he decided to run for President, he asked the Democrats if he could run on their line. They said, sure, why not? They figured he was a long shot, so what the hell?
Why did he decide to run on the Democratic line? It was a pragmatic decision, probably the only one he ever made. He said himself that he could get more exposure than by running as a third party independent. He was right.
Well, that decision backfired on the party and I think it even surprised Sanders himself the way his campaign took off.
Fast forward to now. Sanders seems to be working to destroy the Democratic party. He is running a one man campaign and doing nothing to help elect like minded down ballot Democrats that will help him realize his so-called egalitarian vision.
All he has done is denigrate and call the Democrats corrupt. He is undermining the very party that can help elect like minded representatives.