Alexander Hamilton predicted that the Supreme Court would be the weakest of the national government's three branches. Many would say that things have not turned out that way and would hold up Justice Kennedy's three decades on the Court as an example.
Justice Kennedy was Pres. Reagan's third nominee to replace Justice Lewis Powell, and the fact that Powell's replacement was Kennedy, rather than Robert Bork, would prove to be hugely consequential.
Over the years, Justice Kennedy appeared to "swing" or "switch" back and forth between the Court's more liberal and more conservative blocs, but he held, in fact, a fairly consistent judicial philosophy that prioritized free-speech rights and personal autonomy. Perhaps more than any other justice during his tenure, he believed it was the Court's role and obligation to supervise and second-guess political decisions and resolve social and cultural controversies. His conception of a broad and dramatic judicial role was most famously illustrated in his abortion and gay-rights decisions. In the Planned Parenthood v. Casey decision, he joined an opinion that not only asserted the Court's role as final arbiter but also called on citizens to end their disagreements about abortion policy. Obviously, this call has not been heeded, and Kennedy's critics will say it was presumptuous to issue it.
Other areas where Justice Kennedy's vote made significant differences include the debate over federalism and the national government's powers, religious liberty, church-state relations, affirmative action and voting, school choice, and limits on capital punishment. Both political "conservatives" and "liberals" can find much they welcome, and much they reject, in his 31-year record on the Court.
It seems likely, judging from the current administration's nominees so far, that Justice Kennedy's retirement and replacement change the Court's balance and case-law in some -- but by no means all -- areas. After all, the vast majority of the Court's rulings are not 5-4, ideologically divided cases. Instead, in most cases and for most of the year, the Court works on technical questions of statutory interpretation and operates by substantial consensus. It is, all things considered, unfortunate that the Court and its composition have come to matter so much with respect to disputed and divisive policy and moral questions that our Constitution is best understood as leaving to the political process. It would be a welcome result of the upcoming confirmation battle if both political parties in Congress take it as an occasion to recommit themselves to Hamilton's vision.