Teacher Party ID: Chicken or Egg?

Mike Antonucci asks a great question: what came first, liberals or unions? 

NPR commissioned Ipsos to conduct a nationally representative survey of more than 500 teachers across the country to determine their views on workplace issues.

There is plenty of good stuff if you want to take a deep dive, but I found the responses to this question about political party identification most interesting:

That’s a pretty big swing, but which is cause and which effect? Are unionized teachers more likely to become Democrats, or are Democratic teachers more likely to join a union? Or both?