John McCain has done us all a favor in telling clearly what he would do with the federal court system if he should be elected president. He would continue President Bush’s practice of nominating extreme conservatives to the Supreme Court and lower courts. And he would have plenty of opportunity, since Justice John Paul Stevens, at 88, is expected to retire before long, and there are many lower-court vacancies that the next president can fill.

In a speech aimed at reassuring conservative voters as he prepares for his fall campaign, the often maverick senator flatly described Chief Justice John G. Roberts Jr. and Justice Samuel A. Alito Jr. as models of the kind of jurists he would nominate.

Those two recent additions by President Bush have extended an agenda for conservative change in the high court that started in the Reagan administration. It was shepherded along by the Federalist Society, a little-known group from which many conservative judges have sprung. But the court has remained split between four conservatives – Chief Justice Roberts and Justices Antonin Scalia, Clarence Thomas and Alito – and four liberals – Justices Stevens, David Souter, Ruth Bader Ginsberg and Stephen Breyer. Justice Anthony M. Kennedy has often, though not always, sided with the conservatives.

Among major past decisions that probably would be reversed by one more conservative appointment is Roe v. Wade, the landmark 1973 case that legalized abortion nationwide. Sen. McCain has said twice in recent months that he would favor a high court reversal of that 7-2 decision. The four conservatives have indicated that they favor reversal, and votes and opinions by Justice Kennedy, the swing member, already suggest that he would join them.

Another classic precedent that is in jeopardy is the unanimous 1954 decision in Brown v. Board of Education, which prohibited legal racial segregation in public schools. The Roberts court has turned it on its head, using it to forbid using racial data to promote integration.

In addition, an increasingly conservative court could be expected to continue to break down the separation of church and state, enhance the power of the president at the expense of the legislative and judicial branches, give preference to state rights and limit federal regulation.

How a presidential nominee would deal with the courts is low on the priority list of most voters. In the coming election, the economy, the war in Iraq, terrorism, health care, immigration and other issues will doubtless overshadow any thought of what will happen to the court system.

But federal justices and judges have lifetime tenure. There terms can far outlast the presidents who appoint them. And their decisions can determine the shape of government and affect the lives of all citizens for many years to come.