The Great Slate now has seven races polling within the margin of errror:

https://twitter.com/Pinboard/status/1058835720142512128

(And they have some matching funds, if they've still got any left by now; DM or e-mail or Tweet to get it matched.)

As I understand it, this is pretty much the last point when extra donations will make a difference to anything before Tuesday, so if you have any spare cash left, then they and/or Donate Save America (plus any individual candidates you like) seem like solid choices.