Actually you just need to go read the summaries at 538 to know this is not true statistically.
https://www.washingtonpost.com/news...-democratic-special-election-wave-looks-like/
DailyKos has been tracking special election results since December 2016, comparing the results of the contests with the results of the 2016 election in the same districts. The pattern has been consistent, if not uniform: Democrats have gained, on average, 14 points in their 2016 presidential margins against Republicans in special elections.