Here's my handy cut out and keep guide to effective political campaigning:
When campaigning one needs to realise who one is campaigning to - you're not campaigning to committed supporters of the other side, you're campaigning to uncommitted supporters, or undecideds. Whilst it's unlikely that party-supporting Facebook groups will contain many undecideds, there's a reasonable chance that they might contain uncommitteds.
What this means is that when campaigning - whether that's going into the lair of the other side or what one posts into one's own timeline - you need to craft your message in such a way that it'll appeal to the uncommitted / undecideds.
Calling the other side a bunch of nasty loony idiots, or posting pictures of their leader looking ugly next to pictures of your leader looking inspirational, might make you feel good and get you lots of Likes from your side's supports, but as a campaigning method it's probably the worst thing you can do. Insulting the uncommitteds on the other side is only going to turn them into committeds, and make the undecideds more likely to swing away from your side than towards it.