In my experience there is a SIGNIFICANT degree of seperation between what politicians promise, what they attempt to do, and what actually gets done. Hell, even past behavior is not necessarily of future results. One can even argue there is only minimal skill involved in determining political outcomes by voting for your representatives. Suppose President Obama made himself dictator tomorrow, could any of the Obama voters see that coming? No. Unless you are a direct participant in politics, representative democracy is, for the most part, a crapshoot. Sure you may feel confident that someone like Trump is the right man for the job, but I can guarantee that you will have better odds picking the final four picks than determining what a president will or will not get done. That said, there is a solution to this problem, but that involves in becoming a more informed voter and a bigger participant in government.
In addition, in Obamas first election, McCain seemed unaware of the 2008 economic crash, and despite Obamas seeming awareness it still got to a pretty bad point. Two sides of the same coin.
