Why is it so bad that Obama is moving to the center?

I hear a lot of chatter about how Obama is compromising himself and some of his supporters by becoming more conservative. Some have even called him phony and opportunistic. This irritation is a consequence of his attempt to reposition himself to the “center.” They want him to stick to the liberal ethos; to stay true to the cause. Is that what Democracy is about? It seems to me that he is doing the Democratic thing by adjusting his position to the greatest number. The most fundamental value in Democracy is people rule and if most voting Americans are somewhere in the middle, then Obama is behaving the way a politician ought to behave in a democracy; that is, politicians ought to reflect the values of the citizenry, not impose theirs.