We wrote last week that Catholics may be the ultimate 2012 bellwether as they have bounced back in between the two parties over the past decade and, almost always, wound up voting for the winner.

One of the main reasons that Catholics tend to function as an accurate election predictor is because there are large numbers of them in the upper Midwest, a traditional swing area between the two parties.

Here’s a map released last week from the 2010 U.S. Religion Census that details where across American Catholics live:

That same study, however, shows that the Catholic population has declined in the Midwest (and the Northeast) for each of the last five decades while the religion continues to grow in the South and West. Catholics have also declined by roughly five percent nationwide since 2000, according to Cliff Grammich who helped compile the data on Catholics for the study.