As the Democratic nomination circus heads to Nevada, Sen. Bernie Sanders has run into a problem. Sanders is running on a socialist platform of taking on corporations and billionaires and transferring wealth to the working class. Now, he is being bombarded with attacks from the state’s powerful Culinary Union. The reason: Union workers like their healthcare plans and want to keep them.

The backlash from unions has the potential to blunt Sanders’s momentum in a state he is otherwise expected to win. But more significantly, it’s a massive warning sign for Democrats if Sanders becomes the nominee. President Trump made great inroads with union voters in 2016, and winning them back will be central to any Democratic strategy for beating Trump in 2020.