Looks like Bernie's recent fundraising success has spooked the establishment:
The main super PAC supporting Democratic presidential hopeful Hillary Clinton is making its first significant foray into the 2016 primary, launching a radio campaign in South Carolina and spearheading a $4.5 million effort to drive early turnout … The early engagement by Priorities USA Action — which originally planned to hold its fire for the general election — marks the first major infusion of super-PAC money on Clinton’s behalf.
(emphasis mine)
Of course, according to the Clinton campaign, we should pay no attention to the (wo)man behind the curtain.
Clinton attempted to distance herself from Priorities USA and the donations it has received from Wall Street players, noting the group was first started to support Obama’s reelection. “It’s not my PAC,” she said.