US President Barack Obama has nominated Merrick Garland, an appellate court judge and former prosecutor, to the Supreme Court.
In an address from the White House Rose Garden on Wednesday, Obama emphasised Garland's readiness for the job and history of support from both Republicans and Democrats.
"He has earned respect and admiration from both sides of the aisle," Obama said, praising Garland for his "decency, integrity and even-handedness.
"He is uniquely prepared to serve immediately."
In accepting the nomination, Garland paid tribute to his parents and family, saying: "There could be no higher public service than serving on the Supreme Court."
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Garland, 63, was a former prosecutor who supervised both the Oklahoma City bombing and Unabomber cases in the US.
Garland was confirmed to the DC Circuit in 1997 with backing from a majority in both parties, including seven current Republicans senators.
As appeals court judge, Garland issued an opinion denying Guantanamo detainees from seeking relief in US civil courts.
Before the announcement, Obama sent an early morning email to supporters, saying he had devoted a "considerable amount of time and deliberation to this decision" and consulted with outside experts and groups.
"In putting forward a nominee today, I am fulfilling my constitutional duty. I'm doing my job," Obama wrote. "I hope that our senators will do their jobs, and move quickly to consider my nominee."
That will be a hard sell because Republicans control the Senate, which must confirm any nominee, and the party's leaders want to leave the choice to the next president, denying Obama a chance to alter the ideological balance of the court before he leaves office next January.
Republicans contend that a confirmation fight in an election year would be too politicised.
In advance of Obama's announcement, the Republican Party set up a task force that will orchestrate attack ads, petitions and media outreach.
The aim is to bolster Senate Republicans' strategy of denying consideration of Obama's nominee.
The party's chairman, Reince Priebus, described it as the Republicans' most comprehensive judicial response effort ever.
Garland was twice before considered as a nominee for the lifetime job as a justice but passed over when Obama chose Sonia Sotomayor and Elena Kagan.
Garland had previously served in the Justice Department under Democratic president Bill Clinton.
Garland has won praise in the past from Republicans and Democrats and is viewed as a moderate whose legal approach was shaped by his lengthy career as a federal prosecutor.