The other answeranswers point to more distant historical events, but it's probably more germane to highlight the most recent change that has significantly increased the polarisation of the US Supreme court.
In 2017 the Republican party removed the filibuster rules from Senate procedures around the approval of supreme court justices. This is the so-called Nuclear Option.
On April 6, 2017, Senate Republicans invoked the nuclear option to remove the Supreme Court exception created in 2013. This was after Senate Democrats filibustered the nomination of Neil Gorsuch to the Supreme Court of the United States, after the Senate Republicans had previously refused to take up Merrick Garland's nomination by President Obama in 2016.
Prior to this change Supreme court justices required at least some form of cross part support and indeed many appointments had wide spread support from both parties in the Senate. Justice Ginsberg was appointed on a 96-3 vote despite her liberal credentials.
It is the lack of need for cross party support that has dramatically increased the politicisation of the court.