I don't think you can. Those Christians (Republicans or otherwise) that feel that armageddon is coming do so because they are of the belief that they will be delivered to heaven. Some think that they will be raptured, others simply feel that their reservations are already confirmed. Their actions, deeds, and lifestyles are specifically geared for this. Some religious groups (I've heard of Mormons doing this, other groups like Branch Davidians) actually rely on impending doom to maintain followers, and gather more. If I recall, I recently saw a documentary that showed enterprising Mormons who have stores set up specifically for survival, given the apocalypse.
In short, you're not able to help people that can't see they need it.
Yeah I've seen a few people I know posting stuff like this on facebook. A few where I just couldn't help my self but to ask them if they are just kidding about believing some of this crazy shit because I knew they were too smart to really but into this. I saw an argument about Obama being the anti-christ ... and it wasn't one side saying it was crazy it was one side saying no no... he isn't nearly popular enough to be the anti-christ and saying Obama was an obvious agent of the devil and the anti-christ wouldn't be obvious. I ended up just keeping track of the debate because while very scary It was sort of entertaining to watch these fools go back and forth.