https://www.vox.com/2018/9/4/1676998...cting-midterms
Good article on why tomorrow's vote is so critical.
Right now the GOP has a major structural advantage in the House. It's estimated that the Ds have to win the generic ballot by at least 6 points to secure a House majority. This is because the House is highly gerrymandered for Rs.
Tomorrow many governors will be elected in states currently gerrymandered for Rs. These governors will have the power to approve the redistricted maps in 2020-2021 following the new census. Therefore if Ds win back a lot of the key governerships, they will be able to at least undo the gerrymandered R maps, if not approve maps more favorable to Ds.
Therefore the effects of tomorrow's vote will be felt for at least 12 years in terms of how easy it is for Ds to win the House over those years.