Get used to it, Obama’s whole re-election strategy is to “scream racism” whenever he gets criticized. Simple, but effective.
It takes guts to stand up to that kind of intimidation, and not many politicians have it.
The best defense to that kind of attempted intimidation is to label them before they label you. We live in a sound-bite society and one in which propaganda is effective and ubiquitous. You can't deal with that by responding with dispassionate professorial diatribes.