It depends on when he said if nursing was to help remove or to help further, which could be either. I think he was using words bigger then he can understand to be honest.
Nursing a sense of grievance etc. means to feed one's resentment in order to keep it alive so it is always right there. It doesn't mean "trying to use it for a positive force" under any circumstances. That isn't what he was saying in this sentence so don't try to magically read more into it. Also don't try to mitigate what is being said here or just to rationalize it away -there is absolutely no need for that!
Exactly which of those words in that sentence were just "too big" and led you to believe THIS man couldn't possibly comprehend what he was really saying? Even though everyone reading those words sure understands them. Do you think these are words only white people can understand -but a black man somehow just doesn't really know what he's saying when he uses these words? Your statement that he was using words "too big" to understand is truly MINDBOGGLINGLY arrogant to those reading the words because they are EASILY understandable -and therefore it is also shows a breathtaking and jaw dropping level of condescension toward Obama, a Harvard law graduate I'm pretty sure was saying EXACTLY what he intended here!
You would have been better off defending him for writing it -because THAT I can understand and totally accept and so can most people when such a statement is kept in its original context and not cherry picked out like this. Not too difficult to understand how it might be that the son of a black man and a white woman may not have been able to identify in the least with the race of his mother and reached a point in his life where he was nursing racial grievances and resentments toward those of that race. But even cherry picked out of context like this you can't miss the fact this sentence also suggests there is more beyond just this one sentence.
Obama's and my politics are polar opposites and I can't wait to cast my vote for his replacement next year. But having totally different political beliefs doesn't mean I must assume that is "proof" he is capable of any and all evil though including a deep antipathy toward the overwhelming VAST majority of Americans! (that is far more typical of a liberal to claim those who don't share his/her political opinions do so because they are just plain evil -but it is not a typical conservative stunt) And it also doesn't mean I can't spot a sentence taken out of his book or that I can't spot an autobiographical character explanation when I see it. An autobiographical self-character explanation is an author's discussion about an epiphany of some kind the author believes contributed to making him the man he is today. But in order to explain the magnitude of the epiphany, it typically starts with an admission of bad behavior, irrational thoughts or prejudices followed by the nature of the epiphany that led him to change or abandon them and thus move him in a more positive direction and away from what he NOW recognizes is bad or wrong even though at the time, he did not. This sentence is from his book -and he did indeed go on to explain how he moved past this point in his life.
So to answer the original poster's question about whether he was racist when he wrote this sentence -the answer is NO! He was not expressing any racist beliefs when he WROTE this sentence, it was an ADMISSION he once held such beliefs and behaved in this way. It is an admission that he is able to NOW acknowledge those beliefs and behavior were wrong. Which means he was a racist when he was actually DOING this -but not a racist for his admission he USED to do this. And the other sentences that followed this in his book that were not also copied go on to explain in greater depth what changed for him and led him to realize he should NOT do this. It is an admission that totally humanizes him and maybe for those who think of him as some kind of infallible god that can't be tolerated -but it only proves his shared humanity. With all the faults that implies.