ThePew Forum on Religon and American Life has a nice collection of reports and survey information on religion and politics. See, for example, their profile of African-Americans and religion, or a report on whether a bad economy leads to higher church attendance rates, or a survey on views of the new Pope, Mormonism, and Islam.
What makes this unique is that, unlike with most other demographic information, there isn’t another neutral party gathering it. Which is not so much to say that denominational reporting is flawed, only that it’s gathered for particular purposes. Usually the federal government is *the* source for demographics and statistics (population, occupation, income, education, etc.–see Fedstats), but the Census bureau hasn’t collected information on religion regularly since 1936 or, in fact, at all since a law forbidding census questions about religion was passed in 1976. The parties listed at the link above are all serious and respectable, but tend to be oriented around Christian churches, with much less-coverage of non-Christian religions. That said, the Association of Religious Data Archives has a really interesting interactive map generator which will give you denominational breakdowns down to county level. Or just type in a zip code you’re interested in.
Have a good spring break.