Democratic Primaries

...when he decided to embrace the GOP's racist base by pushing self-deportation in 2012. After that, rather than try to expand their constituency, Republican politicians found that they could capture primaries by doubling down on xenophobia and racism.

On the left, I think the biggest fear with Bernie is that he inspires copycats who embrace the idea of attacking the Democratic Party. While it might be personally beneficial, it results in an extreme and fractured party made up of individuals who cannot think of movement that is bigger then their own self-interests. Bernie is like the Ted Cruz of the left in that he elevates himself at the expense of his colleagues, though Bernie takes it a step further by repeatedly renouncing then rejoining the Democratic party,

So, while we can call out Republicans for helping create the setting for a populist run by Trump, many progressives are drawn to Bernie's populist attacks on Democrats.