The political lean data is mainly based on county-level voting records from two national elections. The source data for political partisanship is based on the presidential election of 2016 (75% weight) and the presidential election of 2012 (25% weight). We will update our profiles to include the 2020 presidential election data as soon as we have access to data that has been approved for external use.
There is a bit of a difference between "is" and "leans" within the political designation on profiles. "Is" denotes an area where the majority of the people are a certain political party compared to average or moderate areas. "Leans" denotes that more people are of one political party, but not the overwhelming majority. This is a little bit confusing to explain, but essentially if a profile says "Is conservative/liberal" it means that a higher number of residents are of that political party compared to areas that "lean" one way or the other.
We are looking to revise our political lean data method in the future because counties may have very diverse political standings.