The European Parliament weighs in on data brokers and data processing in the context of elections in a published answer to a parliamentary question.
“Data brokers may act as controllers or processors depending on the degree of control they have over the processing. Under the General Data Protection Regulation (GDPR)”.
When data brokers process data in the electoral context, they often process special categories of data such as data revealing political opinions or religious beliefs. Such processing is prohibited, except where one of the justifications in Article 9(2) GDPR, such as explicit consent, can be relied upon.