This part: “voters tend to turn against the incumbent party during times of war…” was not accurate for 2002. Indeed, the Iraq War was created primarily to insure support for the Bush II administration out of patriotic fervor. It may have been so in 1966 and 1970 over Vietnam. I am not sure of where in the cycle Lebanon and Grenada affected the Reagan Administration, but 1982 was a terrible year for the GOP, so, concerning that year, you are certainly right. 2002 may have been different because an existential threat was perceived. As for 2018? A man like Trump who despite all his flaws still won the electoral college can “fool some of the people all the time” to enough of an extent to win re-election and help the GOP downballot.