An interesting proxy for this would be the partisan breakdown of Episcopalians in Congress.
There are 19 Episcopalians in the House. 10 Democrats and 9 Republicans. Interestingly enough, it appears there's going to be somewhat of an Episcopalian exodus from Congress later this year: Al Lawson*, Kurt Schrader, Tom Rice, Jim Cooper, and David McKinley have either retired, already lost renomination or are unlikely to win reelection.
The D-R split among other major denominational groups is as followed: Catholics (75-52), Methodists (16-13), Lutherans (7-8), Presbyterians (5-9), Restorationists (0-4), Orthodox (3-4.) Baptists were surprisingly more evenly split than I imagined, 23-31. By far, the largest Protestant denominational group in Congress is "unspecified" with 83 members (26 Democrats and 56 Republicans.)
*I found it interesting that there are currently two Black Episcopalians in the House: Al Lawson and Frederica Wilson. Is the AME church being counted as Episcopal? or are they Black members of the U.S. Episcopal Church?
There are plenty historically black Episcopalian parishes in the US. For instance, St. Mary's Foggy Bottom is a historically black congregation that's existed in Washington DC since the 1860s. Here's the history, which isn't too uncommon:
https://www.stmarysfoggybottom.org/about
