PRRI is listed in many sources as "non-partisan", but then the League of Women Voters is too.
Yes, PRRI is a lefty outfit. But, like all polls, it relies on self-identification of the respondents (as does US government census data). In that sense, it’s equally unfair to all denominations.
There are sources that correlate opinions to intensity of belief and practice. On any given issue, those findings can be compared to PRRI.