I'm Canadian so I don't claim to fully understand the intricacies of American nationality law. The 'natural born citizen' requirement to become the President or Vice-President seems ambiguous to me, but to the Birthers it is not: you have to be born in the USA to become the President and Barack Obama was born in Kenya.
Putting aside the other arguments (he lived in Indonesia! he's a dual citizen! he's a bug-eyed alien from Pluto!) I haven't seen anybody dispute that his mother was not an American citizen. His mother's citizenship would seem to confer American citizenship on Obama.
Mitt Romney was born in the United States (birther alert: his Wikipedia entry doesn't say where in United States that he was born!). George Romney, however, was undeniably born in Mexico to American parents. George ran for the presidency in the 1968. Uh-oh.
Guilt-by-association has become a standard Republican attack. Why have the Birthers not demanded that Mitt Romney disassociate himself from his father and repudiate his father's audacious attempt to subvert the constitution?