“it has become increasingly common in politics to reduce disagreements to bad faith”
This is well observed. But what is the reason? In politics, it is common to offer simple solutions to difficult problems. Such as Brexit against immigration, or bombs against terrorism, etc. Studying the problems quickly shows that these simple solutions are no solutions. So it seems inevitable that those who propose them are either stupid or acting in bad faith. Clearly, the Blairs and Trumps and Johnsons and Camerons are not stupid … so what remains than assuming bad faith?