US President George W Bush has chosen conservative appeals court judge John Roberts as his first nominee to the Supreme Court.
The move could could ignite a fierce partisan clash over Mr Bush's drive to move the closely divided Supreme court to the right.
With Mr Justice Roberts at his side, Mr Bush appealed for a "dignified confirmation process" and a timely vote by the Senate.
Democratic Senator Patrick Leahy
Mr Bush sought to cast Mr Justice Roberts as a bipartisan choice with the right legal background and "a good heart," hoping to avoid the bitter Senate battles that blocked 10 of his most conservative nominees to lower courts during his first term.
At 50 years of age, Mr Justice Roberts could put Mr Bush's stamp on the court for decades to come if he is confirmed by the Senate.
A solidly conservative Republican, he would replace Ms Justice Sandra Day O'Connor, the first woman on the high court and a moderate conservative who often controlled the outcome on issues such as abortion, affirmative action and civil liberties.
Senate Democrats voiced concern about Mr Justice Roberts' record and promised a full review. "No one is entitled to a free pass to a lifetime appointment to the Supreme Court," said Senator Patrick Leahy, ranking Democrat on the Senate Judiciary Committee.
Abortion rights groups seized on a brief he co-wrote in 1990 that suggested the Supreme Court decision that legalised abortion should be overturned.
The American Civil Liberties Union expressed "deep concern" that Mr Justice Roberts, while serving as principal deputy solicitor general from 1989-1993, had backed the criminalisation of flag burning as a form of political protest.
Mr Justice Roberts was part of a three-judge panel that handed Mr Bush an important victory last week when it ruled that the military tribunals of detainees held at Guantanamo Bay, Cuba, could proceed.
The treatment of prisoners there has been criticized by human rights groups and in the Arab world.
The quicker-than-expected decision on a replacement for retiring Ms Justice O'Connor could help the White House deflect attention from a growing controversy over the role of Mr Bush's top political adviser, Karl Rove, in leaking a covert CIA operative's identity, Republican strategists said.
Administration officials said Mr Rove was not a factor in the timing of the announcement.