In the Republicans’ successful 2014 elections, for example, several veteran Democrat senators ran nearly double digits ahead of President Barack Obama’s job approval in their states, yet most still lost.

Eight short years ago, the left also argued that these moderate incumbents would win reelection despite Obama’s declining job approval. Midsummer polls also showed Democrats ahead or even in right-leaning places like Arkansas, Georgia, Iowa, and Louisiana. Those advantages quickly evaporated, and Republican stalwarts like Tom Cotton, Joni Ernst and Bill Cassidy won by large margins in November.

Two years later, no Democrat in a competitive race finished more than five points ahead of Obama’s final 2016 job approval rating. …

It should be no surprise then that this fall’s Democrat Senate nominees, like in the past, have their future tied to the president’s job approval. Senate candidates probably will run a few points ahead of that figure, but that’s no solace when Biden’s approval is hovering around 40%. Unless Biden’s ratings tick up appreciably, Democrats need to win a plurality of votes among Biden disapprovers to have a shot.