There certainly is middle ground on those policies. The US had the Brady Act but still allowed people to own firearms. Most people favor access to abortion with some term restrictions. It's disingenuous to say there isn't a middle ground, particularly when most things in life are a compromise. Moreover, we disenfranchise a lot of people when we say that something is either black or white.
Regardless of what you may think of Lieberman, McCain, Murkowski, Sinema or Manchin, these are the folks that are willing to cross over to the other side of the aisle and make those compromises. They are the big tent people. Why do you think they are villainized in the media? Particularly because they don't fit neatly into one pigeonhole or the other, and more particularly because it's a great ad seller.
We have got to stop thinking we're Republican or Democrat, we shouldn't be aligned to one party at all. We should be voting on the issues of the day, the candidate's platforms and our representatives voting records. Aligning to one party is a poor proxy and it's a cop-out of civic responsibility.