Let's say a POTUS is elected with a friendly SCOTUS and congress behind him. He hates how America's unions and labour rights have been crushed over the decades, and in response passes a very simple yet sweeping bill that mandates membership in a labour union to qualify for employment in a company in the United States.
This was his flagship election issue, arguing that things like the minimum wage, sexism, and even political bribery can be solved more effectively by strong unions (unions can buy politicians too); and more importantly, arguing that mandatory union membership is absolutely necessary, lest corporate America just crush them again.
You're part of the opposition, and you don't like this. Assuming this is a fantasy USA where you get crushed politically if you lie or try anything rhetorically; what can you do to argue against this? What downsides could come from this policy that to an abused labour force sounded like the solution to all their problems?
PS. Don't Nordic countries do something like this already? Tying welfare to union membership or something?