As seen on this useful graphic, Six years after a President is elected, the Senate tends to move against him!
Blue (Wilson) won WH 1912; six years later Senate changed from Blue to Red
Blue (Roosevelt) won WH 1940; six years later Senate changed from Blue to Red
Red (Reagan) won WH in 1980; six years later Senate changed from Red to Blue
Blue (Clinton) won WH 1996; six years later Senate changed from Blue to Red
Red (Bush) won WH in 2000; six years later Senate changed from Red to Blue
Blue (Obama) won WH in 2008; six years later Senate changed from Blue to Red
Obama won in 2012; therefore 2018 was always going to be a bad year for Democratic Senators up for re-election(*). Six years after Trump’s 2016 election — the 2022 election — should be good for the D’s.
The reason is simple: In 2012 turnout was high to vote for President Voters clicked the D Senator while they were voting anyway; some D’s got in with narrow margins; but lost 6 years later in the lower-turnout midterms.
(Also shown on the graphic — or rather, will be shown once it is updated — is that this next Congress will be first since Reagan years with a Red Senate and Blue House.)