After 2016 and the special elections, I now have a rule that if the margin is close, the Republican will always win by default.
Virginia: Gillepsie +2
That said, it only works for close races.
New Jersey: Murphy +14
2012? The national polls were like Obama +1 at best. Granted, that does seem to be the only time in the past 40ish years that the left overperformed fundamentals in a meaningful way (they held both houses of congress in the 1978 Carter midterm, which was legit impressive).
Thus why I said after 2016. The explosion of racial politics during and after the Trump campaign, along with the continued deficiencies of the Democratic party, has led me to conclude that we currently live in a naturally Republican age.