the left should have made it painfully clear that they didn't care about obama's race at all, and was inconsequential to his candidacy.
Not stating an opinion on race issues isn't the same as combating racism. It's clearly the case that race is still a serious issue in the US, or we wouldn't be having this discussion. That won't go away by deciding not to discuss it.
Instead, they said that if you didn't like him, for any reason, you were inherently racist.
This is just painfully wrong. I'm a leftist. I oppose Obama. I've never once been called racist for that. Not even a little bit. I've been asked if I think it's meaningful or significant that a black person was elected in the US (which I do). I've been challenged on specific opinions I hold about his administration. I've never been called racist for opposing him. And almost everyone I know voted for him.
The reason that so many right-wing opponents of Obama are called racist is because there's been an incredible surge in racist organization in the time since he was nominated and elected, and there's a tremendous amount of overlap between that and the right-wing opponents. I don't think it's entirely fair that a lot of run-of-the-mill conservatives are guilty by association in this, but to be fair the vast majority of the reaction to this has been the sort of defensiveness that you've presented here rather than any kind of meaningful denunciation of racism or even simple attempt to understand why race is an issue for anyone other than old white men. Let me make that as clear as I can: people of color also experience racism, and their experience of it is different from yours. It behooves you to understand that. Even if you are not, yourself, racist.
Guilt by association really isn't fair. But it's also not fair to provide tacit political cover for bigots and claim ignorance when it's used to advance bigotry. Hate-related crimes are and have been on the rise, and white supremacist organizations are stronger than they've been for decades. This isn't an accident.
Don't want to be mistaken for a racist? Spend as much time challenging the racists in your midst as you spend attacking people who aren't as stupid as you think.