What are the roots of the current divisiveness of American politics/discourse?

[MarkD]

People feel less agency over the decisions made by government due to increased influence of the SCOTUS.

One of my books about judicial review -- how do the federal courts interpret the Constitution -- is called "Federal Courts, Politics, and the Rule of Law," by John C. Hughes, published in 1995. In the epilogue in the back of the book, it says:
"In the contemporary political context, those who fear conformity have tended to describe themselves as liberal and have tended to applaud judicial 'protection' of human rights. Those who fear diversity have tended to call themselves conservative and have been appalled by judicial 'usurpation' of the majority's discretion to form the kind of community it finds most conducive to its own happiness. The former tends to approve of the expansive theories of constitutional interpretation, ... while the latter tends to prefer the restrained theories of judicial review. These alignments are neither perfect nor inevitable, but the debate has surely been shrill."

The left and the right in the USA do not even have the same theories about how to interpret the Constitution, and when the Supreme Court hands down decisions on issues like public school prayer, abortion, and gay marriage -- decisions in favor of "individual rights" and against "majority rule" -- it emphasizes the differences we have, with one side "applauding" the judges while the other side grumbles about their loss of control by majority rule, and we have shrill debates even about how to interpret the Constitution.