to a Christian, New Testament teachings replace Old Testament teachings.
This is not correct. New Testament teachings do not REPLACE Old Testament teachings. Jesus said (in the New Testament), "Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfil." (Matthew 5:17) Replacing and fulfilling are two different things. We have a saying that goes like this:
"The New is in the Old concealed. The OId is in the New revealed!"
In other words, the truths of the Old Testament are brought to their conclusion and fulfilled there. Therefore, there is much for the Christian to learn from the Old Testament. In fact, there are things mentioned about the Messiah in the Old Testament that are not even intimated in the New Testament. Without the Old Testament we would not have this knowledge.
he has no business make what seems like a volatile attack at obamas personality over a lack of interpretation.
I would disagree for a variety of reasons. One reason is that Obama claims to be a Christian and, yet, his interpretation of biblical passages shows he does not know much about the faith he espouses. Obama, by speaking on the passages of the Bible that he referenced, was reaching out to people of faith (Christians and Jews). However, by misapplying those passages he ends up alienating himself from a decent segment of them and Dobson is just pointing that out.
If I had made a comment about genetics and a geneticist heard my comment and knew that my comment was wrong, he would have every right to correct me. Obama made a quote and comment about biblical passages. Dr. Dobson, a Christian and Bible teacher, heard those comments and has a right to comment on what Obama said. Obama is not a minister (nor does he claim to be). Dr. Dobson does (to some degree) and, as a result, he has a right.