Voting data, party registration data, American Community Survey (ACS) data, and shapefiles for 2012, 2014, 2016, and (soon) 2018 are in the year-labeled folders. Only 2016 data is currently formatted for analysis.
The prep.R script is self-contained; it downloads, reshapes, geographically harmonizes the data. The sov_g16 table (generated by the script) has the voting data; the reg_g16 table the party registration data; and the list of tables called acs_16_prec have all the ACS data (tens of thousands of variables, so split into 122 data tables. Note that the acs_16_template_int file is a codebook for all of those variables). The vote, reg, and ACS data can all be merged with the common ID field srprec. Note that the common geographical unit is voting precinct, not census block as I noted earlier; there are 20k+ voting precincts in California.
The reg and ACS variables will be predictors for the party vote share variable.