From McKay Coppins at Buzzfeed:
The Romney birthers' predominant theory is that because George Romney was born in Mexico, his candidate son is not a naturalized citizen, and therefore Constitutionally ineligible to occupy the Oval Office. (This theory has the unfortunate side-effect of disqualifying President Chester Arthur, the son of an immigrant from Ireland.)
Others believe the younger Romney was, himself, born in Mexico; others point to supposed evidence that his birth certificate is a fraud — and a few even contend that he was raised up by Mormon polygamists to be a presidential plant. But the bottom line for all of them is that Mitt just isn't American enough to be president.