The argument against Obama's citizenship, such as it was, was that Stanley Ann Dunham hadn't been a citizen long enough, because she wasn't old enough, when he was born. (IIRC that inadvertently silly law has been tidied up since.) However, there is no such age requirement for the mother in the case of an illegitimate child. And though Dunham and Obama Sr. were married, the marriage was bigamous and therefore invalid. I have always been delighted by the idea that Obama has to be a citizen in part because he's a bastard.