Okay, I'll just point out Tim Blair's post again here where it's a little more appropriate. It's not just Republicans, in other words, and it's not just in America -- in general, the political right seems happier than the left. As you say, it's not likely that a choice of politics can make you happy (or not), but it's at least plausible that causation works the other way -- happiness (or not) will lead one to make certain political choices.
On a different but related note, thought you might be interested in this post from Tim Blair about a study that seems to correlate self-reported "happiness" with political voting patterns.