A tired meme at the Something Awful forums (and perhaps elsewhere on the internet). Initially it was the funny concluding line of a joke; since then, it has been broadened to be used as a corny punchline for other jokes, and has broadened even further to be added as an addendum to message board posts FOR NO BLOODY REASON (as parasitic memes tend to do)
Used correctly: end of original joke (look it up)
"Why should her laziness and irresponsibility be rewarded with half of what I've worked for?" The father slowly smiled and said, "Welcome to the Republican Party"
Welcome to the Republican Party!
Hey I was just watching the tv and OMG WELCOME THE REPUBLICAN PARTY LOLOL