That's one thing I keep saying to keep myself calm: she probably sees the map just like we do, and is making a calculated bet that the math remains far too daunting for any Republican to win.
First of all there is no guarantee Hillary has a cakewalk to the WH. The GOP has a really compelling group of people. Namely Walker, Haley, Christie, Jeb, Paul, Snyder, Kasich...
On top of this the next two years are critical. If Obama's approval keeps tanking it's going to take a lot of Democratic votes with him. It's foolish to think otherwise.
Democrats literally have Hillary and no one else. Biden will make for some good debate banter but he's not going to win the nomination no matter what.
Outside of that there is also:
-An unprecedented crisis (economic, terror, something) that makes for bad optics politically and impacts people's lives.
-SCOTUS potentially ripping away the subsidy from millions of people which voters will ultimately blame the President for. This hurts Democrats big time if it happens. In fact this might be the one thing that kills their momentum in 2016 with the exception of another crash or a terror attack.
-Gerrymandered EV's in states like MI, WI, OH, FL, maybe even PA before Corbett leaves. They could really get carried away with this. They have a really depressing stranglehold on state legislatures across the nation.
-Democrats continuing to run shitty campaigns and refusing to learn from what Organizing for America taught the party in terms of, like, being able to fucking win.
-Hillary's campaign being an utter disaster. We saw enough of this in 2008 early on; my first impressions of what we've seen from her lately suggests perhaps she didn't learn her lesson well enough.