Close races almost by definition will be affected by poor candidates, by in-fighting among Republicans, and by third-party candidates.
MT, SD and WV won’t be close (so Pressler running as an independent in SD will be of no consequence)
AK, AR, LA and NC may be close. In-fighting could help Begich in AK. LA, with its jungle primary, might require a run-off (which we should win, so this possibility is probably of no consequence). So, Republican pick-up prospects look robust in three of these states with respect to such factors as poor candidates, in-fighting and third-party candidates; and, we’ll have to see what happens in AK.
CO, IA, MI and NH - Recent few elections tell us anything can happen in CO. The Republican field is not yet settled in IA. As a result, the IA race is difficult to handicap at this time. NH has some unique uncertainty with the probable Republican nominee being a former U.S. Senator from MA. So, of these, only MI looks straight-up.
MN, OR and VA - The Republican field is not settled in either MN or OR. The Libertarians look to nominate Sarvis for the U.S. Senate and it is probable the Democrats will again help him to get on the ballot. This would make it even less likely that Warner could be upset. Nevertheless, the Republicans fetched a credible challenger in Gillespie.
An important fact to keep in mind for CO is that the Dem controlled legislature imposed a new, all inclusive, true motor-voter law.
Voters can travel to anywhere in the state they are needed, declare that they intend to move there, and immediately vote in the polling place they have targeted.
Large fleets of buses are being lined up as we speak.