The vast majority of the media coverage of this year’s SuperPAC phenomenon has focused on conservative billionaires who’ve given millions to Republican-leaning or anti-Obama SuperPACs.

But it’d be a mistake to forget about the multi-million dollar investment American unions have made in Democratic-leaning SuperPACs.

The Sunlight Foundation, a nonpartisan group committed to transparency in government and politics, has put together an impressive graphic showing the web of union money in 2012 SuperPACs. It examines the role of four leading labor organizations, the Service Employees International Union (SEIU), the American Federation of State, County and Municipal Employees (AFSCME), the AFL-CIO and the National Education Association (NEA).

Among the findings in the Sunlight report:

– AFSCME has given nearly $1 million to the super PAC Workers’ Voice, founded by the big union umbrella organization — and AFSCME’s parent — the AFL-CIO. In the first two weeks of October, the union gave another $471,254 to the super PAC.

– Both AFSCME and the SEIU have given to Working for Us, a super PAC run by longtime labor strategist Steve Rosenthal. The super PAC has been helping Democrats in several House races and the Connecticut Senate race.

– Both AFSCME and SEIU have given a combined $1.25 million to Majority PAC, a super PAC that’s helping to elect Democratic Senate candidates.

– In October, AFSCME and SEIU also joined forces to back the Sierra Club Independent Action, which has also been involved in several congressional and Senate races.

– All four unions have contributed thousands to American Bridge 21st Century, a super PAC founded by David Brock, a conservative turned liberal activist who founded media watchdog Media Matters. AFSCME contributed the most with $575,000.

For the complete report, click here.