He only ran for President because of Obama, so it's no surprise. For some reason, (and it's not race) he has a unique level of disdain for Obama.
Yes it is. He spent years questioning Obama's legitimacy as the president and as an American.
Well duh! That shifts the attention over to him and provokes controversy. And some people had doubts about the legitimacy of Obama's citizenship.
If the issue with Obama had been over his ears, or because he don't like cherry pie, then that is what Trump would've attacked.
And if the Left had been more vocal about Gitmo and the Public Option being gutted enough for Trump to see the dots, then that is what Trump would've gone after. After all, he had plenty of liberal positions once upon a time.