Not quite equivalent.
Union donations outweigh the other sources, and unlike corporations, which (with some exceptions) tend to be equal opportunity donors (more accurately, they donate to whoever's got the power), unions are almost an appendage of a single political party, with political influence and money flowing easily in both directions. It would be hard to say who's running who.
I also find it disturbing that the unions that control our public education system fall into this category. I suppose you don't.