For US Elections, I'm trying to find the Total Population, Voting Age Population, Registered Voters on Election Day, and the number of Ballots cast, for the years of 2020, 2018, 2016, and 2012, sorted by congressional districts. I've been able to locate some but not all of these fields, and I do understand that 2020 may contain incomplete data.
My advise is to contact the Secretary of State for each of the states that you want data for, as well as what format you need it in. Be clear (and nice) as to who you are and why you are asking for the data. Right now they are busy with certifying the elections so this is a busy season for them, so being polite goes a long way.