If you plan to travel Mexico, you should take the necessary steps to ensure you have proper identification. As with most foreign travel, it's a good idea to bring along your US Passport.
While Mexico will allow you to travel with as little as a voter registration card, your Passport serves as an undeniable proof of US Citizenship and identification. With it, you can enter US Consulates and Embassies if that need should arise for you. Other forms of identification that will get you across the border are:
• A birth certificate
• Naturalization papers
• US issued Green card
• Alien Resident Cards
• Notarized citizenship documentation