At some point during the conversation, you will have to decide whether you want to drag this out tooth and nail-- or just back away. Just like at the bar, or when fighting for a parking space, or at the Department Store during the Holidays-- sometimes it's not worth it.
My advice: back away but not without saying that you would request that people not discuss politics in this setting (family dinner, holiday, whatever)
If they continue to hammer liberal points, go into the kitchen or somewhere else, preferably where there is alcohol.