The PhD electorate are "postgraduates" after you cut out all the MDs, JDs, MBAs and other non-academic degrees. There's also going to be a heavy bias towards folks working in academia/research among PhDs that is less pronounced in the larger "postgraduate" demographic.
Men still outnumber women in PhD graduates. In the STEM fields there's also going to be a heavy bias toward Asian immigrants. All in all, I'd say that PhDs were probably Republican leaning up until the 1960s but have since become a reliably Democratic group. The admission of MDs/JDs/MBAs would move this group considerably to the right, however.
Probably not on the bolded. There are far more JDs (about 1.3 million) than MDs (about 600,000) and I believe than MBAs (but couldn't find data on number of holders), and JDs are a very strongly Democratic group (as I recall, polling by the ABA in 2016 suggested JD holders were voting more than 80% for Clinton - can't find the source right now). I do agree that MDs are about evenly split, and MBAs should be fairly Republican.
Donation information is easier to find; lawyers and law firms donated $34.6 million to Hillary Clinton in 2016
and just $942,000 to Trump (plus a bunch more to other Republican primary candidates, but still way less than the Clinton total in aggregate).
https://news.bloomberglaw.com/business-and-practice/how-much-money-did-lawyers-contribute-to-the-presidential-election/This all aligns with my anecdotal experience of working as a lawyer, too, which is that Republican lawyers, even at the highest income levels ($1m+ annually), are a relatively small minority.
