On the final day of oral arguments last term, the chief justice’s voice cracked with emotion as he bade farewell to the retiring Justice Stephen G. Breyer. It was a striking moment for the normally buttoned-up John G. Roberts Jr., and one that seemed to signify more than sorrow at the departure of a longtime colleague. It is not far-fetched to imagine that Roberts was mourning the decisive end of his vision of presiding over an institution seen as operating above the partisan fray. “I’ve lost my only friend on the court,” Roberts told someone afterward.