That's why they call them "the enemy brothers", Walter.
The PRI was first to belong to the IS, even if it is -in Marxists terms- a Bonapartist party (counter-revolutionary inside the revolution).
PRD was born in 1989, as a big fusion of the remnants of the old Socialist Unified Party, an important group of disgruntled priistas who thought their party was moving too much to the right and assorted smaller groups or splinters.
Every flag waving Mexican has a little priista inside of him/her, just (as I imagine) every flag waving Russian has a little Bolshevik inside. It's a love/hate attitude.
A few months ago, one of Calderón's cabinet member's cellphone was tapped, and he was caught saying: "I swear I miss PRI". He resigned a few days later.
In the 80s, we were watching the news -the usual Latin American turmoil and riots- and my small son asked me: "Why doesn't any of that happen in Mexico?". I was amazed at my answer: "Because we have PRI".
The PRI leader says her party is "progressive". I concede she is a progressive, but the party is a big mix of interest groups.