Notes on the data from
www.polidata.org:
"Only a few states compile the results of the presidential election by congressional district. Therefore, data must be collected from local election offices. This normally means precinct-level election results. There are 450 counties that include a portion of more than one congressional district. Also, due to the presence of split precincts and centrally counted absentee and early votes, there may not be a hard-and-fast number for some districts and there may be slight variations amongst the few sources for these data.
"There are only a handful of projects that bother to undertake this months-long project. While commercial enterprises are involved in these projects, this is not a big money project. POLIDATA has been involved with the development of these data since 1984."