The Democrats are totally ill-equipped to defeat him. He simply doesn’t fall for their game, and so it will require the combined efforts of Democrats, Republicans, Libertarians, the American Communist Party, et al, plus Jihadists and other foreign interests, to defeat him. This will be interesting.
The predictable, or rather a common, outcome would be that someone would get to him and convince him (threaten, bribe, cajole, lie, intimidate, mesmerize, etc.) that “For the Greater Good of the Party” he should quit before he does too much damage to the 2016 prospects. That or they primary him right out.