Usually when polling data is released they’ll have info about the political affiliation of voters. I don’t know about this election, but many of the special or off year elections since Trump have shown a mixture of lower republican turnout and a higher percentage of republican voters voting for democrats.

But for exact numbers you’d need to look into the elections. The Virginia state elections would probably be most telling as they do elections on odd years, so it wasn’t a special election and was for the whole state. Virginia is also a “purple” state and ended up flipping republican to Democrat majority.