One obvious place to draw the line - indeed, the place where used to be drawn - is the part where Citizens United tried to pay channels to air their film about

One obvious place to draw the line - indeed, the place where used to be drawn - is the part where Citizens United tried to pay channels to air their film about Hillary. Normal media outlets are businesses, which means that they need to cover their costs somehow. They also have to gain and keep the trust of their audience through their reporting. If you're just paying off existing media outlets to show your propaganda to their audience solely for your political gain, then you're engaged in campaigning. This is why Citizens United's earlier attempts to get Fahrenheit 9/11 and its trailers banned failed; the FEC found that it was bona fide commercial activity.