President Barack Obama is nominating veteran appeals court judge Merrick Garland to be the next US Supreme Court Justice.
The Supreme Court vacancy follows the death of Antonin Scalia last month.
Judge Garland, 63, is viewed as a moderate and has won praise from senior Republican figures.
The appointment has to be ratified by the Senate, but its Republican majority earlier vowed to block a vote on any Supreme Court nominee from Mr Obama.
Republicans have called on Mr Obama to leave the nomination to his successor, who will be elected in November.
The death of Justice Scalia, a staunch conservative, left the nine-member Supreme Court evenly divided between conservatives and liberals.
It also set off a battle in a presidential election year over his successor.
Urging the Senate to support Judge Garland, the US president said: "He is the right man for the job. He deserves to be confirmed".
President Obama said Judge Garland - chief judge of the Washington appeals court and a former prosecutor - enjoyed respect from Democrats and Republicans alike.
Announcing the nomination in the White House Rose Garden, Mr Obama praised Judge Garland's decency, integrity and even-handedness during his long career in public service, and described him as an exemplary judge.
His nominee was prepared to serve on the court immediately, he said.
President Obama expressed hope that Republicans would act in a bipartisan spirit and give the judge a "fair hearing".
The nomination was the "greatest honour of my life", Judge Garland said.
The judge was appointed by Democratic President Bill Clinton in 1997, winning confirmation in a 76-23 Senate vote, and served in the Justice Department during the Clinton administration prior to that.