Mexico's politics in the 20th Century were remarkably boring by Latin American standards. As the rest of the region lived through coups, civil wars, and revolutions, Mexico stayed quiet from 1929 to 2000 under the one party rule of the PRI. So let's make things a little more interesting. Can Mexico have a marxist insurgency movement similar to FARC in Colombia or the Shining Path in Peru.
Well, Mexico packed all the excitement any nation could handle into the first three decades of the 20th century.
Also, this happened, basically the North American equal to Tiananmen Square :
https://en.wikipedia.org/wiki/Tlatelolco_massacre
Anyway, a communist insurgency/takeover in Mexico is the stuff of bad Red Dawn fan fiction. The US would probably intervene in Mexico directly (even in the post-Vietnam malaise) and it would probably be a bloodbath, especially since a competent insurgency would be able to coordinate retaliation strikes against the continental US (i.e. the inevitable My Lai or bombed out village is answered with IRA-style car bombs, etc...). Also, given that the Cold War is likely still going on, one wonders what the Soviets would do, especially if they were fighting in Afghanistan at the time. A bit of tit for tat would be tough to resist, so you might see an insurgent with some quite nice weapons.
It would be a massive clusterfuck.