A dem president over 2 terms will probably lose 600 seats, the average loss in a 1st midterm alone is 420 legislative seats for the party that controls the WH, the GOP only lost 295 in 2018 because they were able to outperform historical trends and minimise their losses owing to Trump's strength outside urban areas, in many states they basically lost almost no seats, in WI they had a net gain of 1 legislative seat and in Iowa the net loss was only 2 legislative seats.
I think the 1st midterm for a dem president will be closer to the average loss of 420 unless they can expand their base outside urban areas, there are still hundreds of dems in legislative seats in areas that are trending Republican that will be wiped out the next time there is a midterm under a democratic president.
https://ballotpedia.org/State_legislative_elections,_2018
