An over-simplified statement of the thesis is: Mitt Romney’s father was born in Mexico, making him a Mexican citizen and under Mexican law Mitt inherited his father’s Mexican citizenship. A counterargument says that this theory is OK today, but that the Mexican Constitution has been changed a couple of times since George Romney’s birth, and it’s not clear whether the newer provisions are retroactive.
While this speculative discussion is entertaining, it strikes me as profoundly silly in the context of deciding whether Mitt Romney is eligible to run for President of the United States. Why should nitpicking analysis of the Mexican Constitution matter in the United States? Mitt Romney is not running for President of Mexico.
This same type of argument has been raised in all seriousness by the birthers against Barack Obama, with folks like Mario Apuzzo delving into the various changes in the British Nationality Act to try to show that Obama is a dual citizen of the UK and the US. Barack Obama is not running for office in the UK, any more than Mitt Romney is running for office in Mexico. These examinations of foreign nationality law should have no relevance as to who can be President of the United States, with the sole exception of the fact that the Constitution itself used terms of art borrowed from English Common Law, and so “natural born citizen” is defined by that context.