Maybe both. But the organized campaign is likely far bigger than the number of radical leftist advertisers.
Yesterday, I got an email from one such outfit: LeftAction.com (how I got on their list, I don't know). They're running a boycottRush.org campaign. A bunch of rabid, leftist weasels. I unsubscribed to the list and sent them a blistering email.
Almost every tweet since March 1 is about Rush Limbaugh.