> How would you even collect data about this?

You gather the data required to make a good probability prediction for voter preference ((soft) labels for this easier to find than swing voter labels). Then when the model is uncertain, those are your swing voters / on the fence voters.

> Postcode? Age? Race? Gender? Income?

When it is found to be cost-effective: All and everything that is allowed by law and then some. In its pitch deck, Facebook boasted about its advertisers being able to target and identify: university, degree, concentration, course history, class year, housing/dormitory, age, gender, sexual orientation, zip (home and university/work), relationship status, dating interests, personal interests, club membership, jobs, political bent, friend graph, site usage/addiction level.

Likes make this very easy (with a little luck, you can deduce all of zip, age, race, gender, income from a list of Likes).

> What is it about CA's methods that were so effective?

Hillary Clinton: “The real question is how did the Russians know how to target their messages so precisely to undecided voters in Wisconsin or Michigan or Pennsylvania – that is really the nub of the question. So if they were getting advice from say Cambridge Analytica, or someone else, about ‘OK here are the 12 voters in this town in Wisconsin – that’s whose Facebook pages you need to be on to send these messages’ that indeed would be very disturbing.”

FBI: Using those techniques in June 2016, “the GRU compromised the computer network of the Illinois State Board of Elections by exploiting a vulnerability in the SBOE's website,” the report said. “The GRU then gained access to a database containing information on millions of registered Illinois voters, and extracted data related to thousands of U.S. voters before the malicious activity was identified. Similarly, in November 2016, the GRU sent spearphishing emails to over 120 email accounts used by Florida county officials responsible for administering the 2016 U.S. election,” the report said. “The spearphishing emails contained an attached Word document coded with malicious software (commonly referred to as a Trojan) that permitted the GRU to access the infected computer.”

> After all someone had to do something similar for Obama.

Obama's digital campaign was very successful, but the above seems to indicate that Kushner's campaign was way more aggressive and less scrupulous (and may have had connections with - or help from foreign adversaries).

It may also be that propaganda and smears works better depending on your political preference and level of education and neurosis: Even if Hillary had spent the same amount of money and energy (some reports indicate that Hillary's digital campaign was a waste of money and displayed poor management), efficiently, it may be easier to sway a voter to vote Republican, if you can target their fears of immigrants, religious beliefs, distrust in gun regulation from the government, and conspiracy theories. Surely, the many wolf cries about fake news, and retweeting of conspiracy theories, has set up the Trump base for easier manipulation (you can simply create a meme to counter a story in a respected journal or keep them guessing on the alternative truth of it).