Our new book is titled Divided We Stand: The 2020 Elections and American Politics. Among other things, it discusses state and congressional elections.
When you look at the first 17 special elections this year (through early April), the Republican overperformance over Trump was just a point. Examining the last 17 special elections, the overperformance has been 7 points. When you splice the data even further, Republicans have been outperforming the 2020 baseline by double-digits since the beginning of July.
Whether such a shift sustains itself can't be known at this point. Things may shift back to Democrats.
It shouldn't be too surprising if the Republican overperformance does hold. Back in 2009, there was a big movement away from Democrats in special elections toward the middle of the year. This foretold Republicans doing very well in the 2010 midterms.
The one thing that is clear is that the special elections this year look nothing like they did four years ago at this point. Back then, Democrats were outperforming Hillary Clinton's performance by 14 points in the average special state legislative and federal elections.