Elena Kagan said she was honoured and humbled by the nomination
US President Barack Obama has nominated Solicitor-General Elena Kagan as the 112th justice to the Supreme Court.
Ms Kagan, a 50-year-old former Harvard Law School dean, was at Mr Obama's side at the White House when he announced, as expected, that she was his choice.
She would be the youngest member and third woman on the current court, as well as the first justice in almost four decades not to have been a judge.
Republicans warned she would not receive automatic approval.
The Senate must confirm whether the nominee - who has spent much of her professional life in academia - can replace retiring Justice John Paul Stevens, who is considered one of the court's leading liberals.
Gay rights advocate
Democrats welcomed the nomination, but Senate Republican leader Mitch McConnell said Ms Kagan's "brief litigation experience" would be reviewed.
The BBC's Richard Lister in Washington says Ms Kagan is regarded as a liberal on most issues.
But she has worked with conservatives and even attracted a liberal backlash against her support for continuing Bush administration policies on state secrets and the use of military commissions to try terrorism suspects, our correspondent adds.
Her staunch advocacy of gay rights may concern Republicans.
She went through a fairly smooth confirmation process in the Senate for her current job this year, when seven Republicans voted for her.
With 59 votes, Senate Democrats would have enough to confirm her, but they would be one short of being able to halt blocking or stalling tactics by any Republican senators.
Early in her career Ms Kagan was a clerk for a US Court of Appeals judge and later for former Justice Thurgood Marshall.
And like Mr Obama, she worked on the prestigious Harvard Law Review as a student.
Our correspondent says that selection of a Supreme Court justice gives a president the opportunity to influence US public life for decades, as the justices are not bound by term limits.
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