And 90% of union PAC money is spent on Democrats.
A union response to your claims:
There are strict legal limits that help make our political efforts transparent. Most of our political funding comes from SEIU COPE, which reports its donors on a monthly basis, which cannot accept more than $5000 a year from any one donor, and whose donors overwhelmingly are a hundred thousand low wage workers contributing around $10 a pay period. To the extent we do political work funded by our general treasury, most of which is member to member work funded by and accountable to those same low wage workers. We don't - and can't - solicit contributions from non-members. And of course it is disclosed.
The Chamber and the shadowy 527 and c4 groups that have sprung up after Citizens United - perhaps more aptly called corporations united - are conduits for undisclosed corporate money, pure and simple. We are a union of working people, and the money we spend on politics is money donated by workers. Their attempt to liken us to them in this regard is at best ignorant and frankly wrong.
Union disclosure unlike other groups' - Ben Smith - POLITICO.com
Why won't the COC disclose?