Citizens united opened the door to unlimited corporate spending on campaigns. However, corporations don't want to be seen as political, because they're not interested in red or blue dollars, they'd rather stick with the green stuff. So after Citizen's United, the primary use of 501(c)(4)s in poliitcal campaigns was to evade the financial disclosure requirements of 527's, while masqurading as public welfare groups. This was done by both left and right wing groups, but it was roughtly an 80/20 split, conservative vs liberal.
During 2010-2011, there was a tremendous outcry by campaign finance watchdog groups, (on both sides) to investigate behaviour that was clearly not in keeping with the purpose of 501(c)(4) organizations. The IRS tried to comply, but did it incorrectly, profiling the groups most likely to engage in illicit behaviour.
I'm not exactly sure about your facination with the so called "progressive machine" (whatever that is). But seriously, If you can find it.. please take it out back and put it out of its misery. If such a thing exits, why couldn't it managed to get universal backgroud checks passed with the presidency, the senate, and with 90% public support. It's like striking out at T-Ball... in a league that doesn't count a miss as a strike.