The Guardian has an excellent piece on recent moves by Facebook and Google in seeking to ban micro-targeting political ads.
The practice, which underpins the Cambridge Analytica scandal, is being reviewed by the news providers. The harvesting of political oriented data is common and is usually undertaken as part of an effort to profile users.
Facebook has been known to group users for the purposes of ads-targeting, some of which considers political interests. This allows for nuanced and in many cases, an alarming degree of differentiation and influence of users. The US legislatures have taken issue with this approach in the past.
The ICO has recently reached an agreement with Facebook over the fines put in place over the Cambridge Analytica scandal. The regulator continues work into data misuse in political advertising, to which the issue of micro-targeting of political ads is central.