My personal experience is that there are plenty of people on the right or center-right who despise centrist Democrats moreso than they do progressives. Like, my parents are conservative Catholics who would never dream of saying a kind word about someone like Clinton, Biden, Pelosi, or Schumer, but with Sanders they said things like, "I disagree with him on abortion but I do think he's trying to help the country." Even my brother, who watches shit like OAN, criticized him for "not actually being that different from the rest of them." The mainstream, centrist wing of the party is a known quantity and they hate their guts, they don't agree with or trust progressives, but if they had to pick they'd rather roll the dice.

People think of politics as being a linear spectrum of left-right but for most people it's more complex than that. Some people are able to see people like Schumer for the slimeball that he is, but for whatever reason they might have a blind spot for the other side's flaws.

No idea how relevant that is to your father-in-law but I'd wonder if he even knows anything about Van Hollen other than that his name isn't Chuck Schumer.