The rule of law is the cornerstone of constitutional democracy - and nowhere more so than in the United States. President Bush's decision to nominate Mr John Roberts to replace Justice Sandra Day O'Connor on the US Supreme Court is a prompt response to her retirement and a shrewd selection given the highly political character of the forthcoming ratification hearings in the US Senate.
Mr Roberts is a political and judicial conservative, but in the mainstream of his profession rather than from the ideological margins. His relatively short career as a federal appeal court judge means there is much for the Senate to probe. Given the central importance of the Supreme Court in the US political system and the likelihood of further resignations from it during Mr Bush's second term this will ensure a huge audience for them. Constitutional law is the coping stone of the US federal edifice. There is a standing argument between conservatives who believe the court's task should be limited to a strict interpretation of the Constitution's original text and liberals who say it is a continually evolving document requiring fresh interpretation in the light of changing political and social realities.
The balance between conservatives and liberals was given a certain stability by Justice O'Connor's willingness to side with the liberals, on affirmative action and abortion for example. Observers believe Mr Roberts would be more consistently conservative. His record on such issues involving federal law on environmental regulation, child labour and safety at work will be scrutinised closely.
US political history has involved a continuing tussle between the functional centralisation which produced its welfare and social protection systems in the 1930s and 1960s, or the huge expansion of military and foreign capacity after 1945, and the countervailing decentralisation demanded by conservatives to reassert states' rights. The Supreme Court often has to mediate such conflicts, which gives it a quasi-legislative role.
Mr Roberts is very much an insider, having presented 39 cases before the court, in most of which he was successful. His nomination has been welcomed on the Republican right. Mr Bush calculates that the Democrats will accept him, given that he received unanimous Senate approval as a federal appeal court judge.
Inevitably there will be close attention to his views on abortion, which are ambiguous. He presented cases critical of the landmark 1973 Roe vs. Wade judgment during the Bush snr years, but has since said it is the settled law of the land.
Mr Bush's decision to nominate a mainstream rather than a radical conservative is calculated to regain political credibility shaken by the Karl Rove affairs and the Iraq war. But this will probably not be his only opportunity to change the balance of the Supreme Court for years to come, given the age profile of its older members.