President George Bush has nominated a close aide, White House counsel Harriet
Miers, for a key Supreme Court vacancy.
If approved by the Senate, Ms Miers - who has never sat as a judge - will
take up the place left by Sandra Day O'Connor, who stepped down in July.
Democrats are likely to fight the nomination if they think it tips the court's balance towards the right.
Ms Miers, 60, said she was humbled. The Supreme Court is one of the most influential bodies in US public life.
Mr Bush's announcement came as the Supreme Court prepares for its first day of work after the summer recess.
The nine-member court will be considering some of the most bitterly contested social issues in the US, including assisted suicide, abortion, same-sex marriage, human cloning and campaign finance law.
Precedent
Sandra Day O'Connor was considered the holder of a key swing vote on the court,
which is why the new nomination will be keenly scrutinised.
The appointment of a conservative judge could tip the balance of the court.
Speaking from the Oval Office, Mr Bush said the Texan-born lawyer Harriet
Miers had "devoted her life to the rule of law and the cause of justice".
"She will be an outstanding addition to Supreme Court of the United States,"
he said.
He also named other justices who had been appointed to the Supreme Court without previous experience on the bench, including the late Chief Justice Rehnquist.
Republican Senate Majority Leader Bill Frist said Mr Bush had picked "another outstanding nominee".
"Ms Miers is honest and hard-working and understands the importance of judicial restraint and the limited role of a judge to interpret the law and not legislate from the bench," he said.
The BBC's Oliver Conway in Washington says Mr Bush appears to have reached out to the middle ground, by picking a woman and by apparently consulting with Democrats before making his decision.
But it is still not clear whether or not the Democrats will try to block Ms Miers' confirmation.
'Assertive'
The Senate minority leader, Democrat Harry Reid, who voted against John Roberts
as US chief justice in his confirmation vote last week, said he liked Ms Miers.
"In my view, the Supreme Court would benefit from the addition of a justice who has real experience as a practicing lawyer," he said.
But Democrat Charles Schumer, a member of the Judiciary Committee, suggested Ms Miers' lack of a judicial record could cause problems.
"Americans will need to know a lot more about Miers' judicial philosophy and legal background before any vote for confirmation," he said.
Some conservative groups have expressed disappointment with her nomination, saying they had been promised a staunchly conservative candidate.
"The President's nomination of Miers is a betrayal of the conservative, pro-family voters," said the conservative advocacy group Public Advocate.
Ms Miers, 60, has been serving as President Bush's top legal counsel since November 2004. She also served on Mr Bush's gubernatorial campaign in 1994 - and again during his presidential election of 2000.
Friends and colleagues describe her as assertive and ambitious, while being discreet and selfless.
Monday also sees Chief Justice John Roberts, a Bush appointee, take up the helm of the Supreme Court for the first time.
He had originally been picked to replace Ms O'Connor. But when Chief Justice William Rehnquist died last month, Mr Bush decided to nominate him for the top job.
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