User Guide and Data Dictionary. (In PDF)
From the Introduction:
The Office of the Clerk makes available membership lists for the U.S. House of Representatives. These lists are available in PDF format, and starting with the 114th Congress, the data is available in XML format. The document and data are available at http://clerk.house.gov.
For unknown reasons, the link does not appear as a hyperlink in the guide. http://clerk.house.gov.
Just as well because the link to the XML isn’t on that page anyway. Try: http://clerk.house.gov/xml/lists/MemberData.xml instead.
Looking forward to the day when all information generated by Congress being available in daily XML dumps.