Another problem is that both parties are adamant that they're right. I admit, rarely has my mind been changed. But more importantly, whereas we go into a debate with a liberal thinking they're wrong, they go into it thinking we're evil (i.e. bigoted, homophobic, racists, etc.). We can't successfully debate them. They have anger towards us.
Now I know this isn't the case with all liberals, but I strongly believe that it's common; certainly, with most I've debated. So, you can bang your head against the wall if you want (I'll continue to do it), just know your facts don't mean a thing to a liberal.