From your link:

"Some of the biggest names in the private prison industry have given $835,514 to federal candidates since 2000 and a stunning $6,092,331 to state politicians in the last five elections cycles, according to data in the report."

That's $6,927,845 since 2000. Call it $7 million. All of the private prison companies, across the whole country. Hold that number in your mind.

Now tally up what the CCPOA, the California prison union, has spent on political activities in that time period. One union. In one state.

I get a total of about $33.6 million. By one union, just in California.

We done here?

I had looked at the Justice Policy Institute paper before I made my first response. I do the research first, you see. No mention of union political influence anywhere, which is not surprising considering the source. There is a mention of unions, though. From that paper:

Most private prisons do not allow the formation of correctional officer unions, which helps to reduce the overall cost of running a private prison, but limits the staff’s ability to negotiate pay, benefits, and proper training. Although proponents of private prisons argue that unions drive up prison costs, they appear to offer a level of stability and training that is not present in most private prisons.

And there you have the entire foundation of this anti-private prison deal. Exact same reason the Left hates Wal-Mart - they aren't getting that union loot. The CCPOA hauls in $22 million a year in union dues. Again, that's just one California union.