I don't thing the Democrats were worried that Cain might beat Obama.
I would suspect someone in the Republican Party.
But my point is that when a politician is vulnerable, that vulnerability can be exploited with money.
Someone was paid to listen to all of Rev. Wrights sermons before they could "expose" President Obama from listening to one of them. No one proved that he actually did hear that sermon, by the way. Someone paid the Swiftboaters as well. Linda Tripp was paid. Tese were not accidental occurrences.
Carson will not be the nominee. At the present time, he is no real threat. but I imagine that there are some dirty tricksters at work on him, and Trump and several others.