It’s not hard to understand at all, Democrats unite against anything Republican especially when Trump is involved and Republicans are split over Trump
Trump is by far the dominant Republican and his endorsement is key in Republican primaries but works against the candidates in the general election
Just look at 2022 in a number of Senate races Georgia and Pennsylvania plus state races in Arizona are prime examples
Oh Horsesh!t.
Georgia, Pennsylvania had horrible candidates. Arizona was a bit better but not enough to get over the line.