Start here, at Bob's blog: http://thebusmansholiday.blogspot.com/2009/03/peduto-campaign-finance-reform-by.html
The loophole is PACs.
A union's PAC can give 2.5 times as much as an individual. But, a union PAC often has a city, county, regional and national PAC. And, it can have Grey PAC, Gay PAC, At-Home PAC, Retired PAC, Women's PAC, and so on. Anyone can make a PAC. PAC can then funnel money to candidates.
A rule follower could give $1-M to various PACs and they can all end up giving money to the same candidate for the same campaign.
When the rights of a mob, group or clan exceed that of the individual, red flags should fly.