That's the point, you can talk about it with your dad, but not with your lesbian aunt, your single-mother cousin, your autistic little brother, and your gay uncle sitting there too. If you do that, you're just guaranteed to start a fight and ruin Christmas.
Better to keep your power level low and thank God you don't have to see these people very often, than starting a family free-for-all that NOBODY will thank you for, and that will accomplish NOTHING. If you already know they're libtards, there's no point in talking to them about politics en masse. You won't change their minds and the whole thing will devolve into an ugly argument for no benefit. Save political arguments for when there's a chance of some benefit, and those chances are far better in one-on-one conversations than in a group setting.
Besides, nothing stops a liberal rant faster than a chilling silence. Arguing feeds their victim complex, but failure to acknowledge or reward a purity spiral stops them cold.