Corey Stewart. He obviously had no chance regardless, but his campaign was a hot mess that seemed to be doing its best to make his loss margin as high as possible and seemed to think he was running in West Virginia. Come to think of it, he might've actually done better than Morrisey if he did, lol.
Honorable mentions to Bill Nelson, Debbie Stabenow, Jim Renacci, and Lou Barletta who sleepwalked through the campaign, Dean Heller and Leah Vukmir who seemed to think they were running in Oklahoma (must've read too many Atlas posts/pundit columns about their respective states), Heidi Heitkamp who literally leaked the names of sexual assault victims right before the election and seemed to think she was running in Vermont, and Patrick Morrisey and Leah Vukmir for just being awful and unlikable in general.
I agree with most of this, though I think Morrisey takes the cake for blowing what should have been an easy pickup. There was a lot to attack Manchin on: his pharmaceutical ties and his daughter’s company, his sketchiness as WV Governor, his numerous about faces, etc.
Also I don’t think Heitkamp campaigned as if she was running in Vermont. She was pretty vocal about being one of the most pro-Trump Dems in the Senate and she still got destroyed. She was doomed from the get go just by running in ND