The Washington Post reveals two distinct lines of attack running simultaneously on Sanders now that HRC is pushing the panic button (1) He's just telling voters what they want to hear but can't deliver (link) (2) he's unelectable because his ideas are so radical that voters will find them repugnant. (link)
Those two lines of attack are obviously in some tension, although I guess, to be fair, the first is being applied to Democratic primary voters and the second to the general electorate. Still, the argument that his policies are shiny attractive baubles for the Democratic base and utterly repugnant to the portion of the electorate that would be in play for deciding the election strikes me as an implausible claim.