From TNR's Noam Schieber, an excellent, comprehensive look into the potential of a Clinton vs. Warren race in 2016. Indeed it's too comprehensive to efficiently compress. I'd suggest spending a few minutes with the original.
The story does, however, have one clause worth noting here--one that is nominally unrelated to a Clinton-Warren showdown, although it perfectly portrays Hillary's underlying danger in any contest: "Clinton-era nostalgia is always a news cycle away from curdling into Clinton fatigue."
Just a few months ago I regarded Hillary as about as sure as a presidentially sure thing can get. My confidence level is now more like 98, not 99, percent.