You're unlikely to convert someone like this, but you might be able to make him feel like an idiot for spending his whole Xmas letter talking about politics. Unless the people on his card list are diehard leftist political junkies, they are not going to care to read his ranting diatribe in a venue where such things are best left out.
I don't know about anyone else out there in Freeperland, but if I wrote the conservative equivalent of that letter and sent it out in my Xmas cards, my family and friends - even those who agreed with me - would be very put off. There's a time and place for everything, and a holiday card is really no place for a manifesto (especially one where you try to project your own political opinions onto Jesus!!!)
Just my $0.02
And, meanwhile, all we learned about his kids is that they got to watch videos in the van during their summer vacation. My kinda Dad (Not!). Jeez!