As long as you are a u.s citizen with a us passport you are considered essential to enter back into the states.
Originally Posted by Batsy
You are allowed to re-enter the US if you are a citizen, even without a passport. The passport just makes it easier to verify. You are perfectly fine returning to the US without a passport, but they may take a while to verify your identity. I've seen them send people to secondary and take hours to confirm their identity, even with a drivers license and birth certificate. I've seen them hold people just long enough for them to miss their plane. But legally if you are a US citizen, they have to let you in.
Mexico has no restrictions to enter.
Originally Posted by Batsy
Some Mexican officials are requiring a passport to enter, and some aren't. Without a passport it's possible that they will ask for a "tip" to allow you in. Either way, they may require that you pay $30 for a 180-day permit called an FMM (Forma Migratoria Multiple), even if you are only staying for a short time. The form used to be free for stays under seven days.
In January, I showed up with a passport and my FMM already filled out (but not paid for). They let me in with no problems without paying. When I went back in February my passport was being renewed so I only had a photocopy of it, plus my Drivers License and the FMM. The guy said that a photocopy was not enough. He tore up the FMM and said I couldn't enter because travel was restricted. I told him I had a doctor's appointment and showed my doctor's business card with the date written on it. The guy said I could go through if I gave him a "tip". Five minutes and 400 pesos ($20) later I was in Tijuana.
There are no absolutes when dealing with Mexican officials.