U.S. President Barack Obama has nominated appeals court Judge Merrick Garland to the Supreme Court, challenging Republicans to reject a long-time jurist and former prosecutor he called "one of America's sharpest legal minds."

Garland, 63, is the chief judge for the United States Court of Appeals for the District of Columbia Circuit, a court whose influence over federal policy and national security matters has made it a proving ground for potential Supreme Court justices.

He would replace conservative Justice Antonin Scalia, who died last month, leaving behind a bitter election-year fight over the future of the court.

Obama announced the choice at a ceremony in the Rose Garden, with Democratic Senate leaders and allies looking on.

Garland said there could be no higher public service than serving on the Supreme Court.

Obama held up Merrick as diligent public servant, highlighting his work leading the investigation into the Oklahoma City terrorist bombing. He quoted past praise for Garland from Chief Justice John Roberts and Utah Republican Senator Orrin Hatch. And he said Garland's talent for bringing together "odd couples" made him a consensus candidate best poised to become an immediate force on the nation's highest court.

Garland is known for his "decency, modesty, integrity, even-handedness and excellence," he said.

Obama noted Garland was confirmed to the D.C. Circuit in 1997 with backing from a majority in both parties, including seven current Republican senators.

Senator Chuck Schumer of New York, the No. 3 Democratic leader, called Garland's selection "a bipartisan choice."

Senate Minority Leader Harry Reid, who spoke to Obama Wednesday morning, said in brief remarks on the Senate floor that Republicans must act on the president's choice. "He's doing his job this morning, they should do theirs," said the Nevada Democrat.

If confirmed, Garland would be expected to align with the more liberal members, but he is not viewed as down-the-line liberal. Particularly on criminal defence and national security cases, he's earned a reputation as centrist, and one of the few Democratic-appointed judges that Republicans might have fast-tracked to confirmation — under other circumstances.

Senate Majority Leader Mitch McConnell said the American people must have a voice in November on filling the Supreme Court vacancy.

In a speech on the Senate floor on Wednesday, the Kentucky Republican made it clear that the GOP-led Senate will not consider Obama's nominee, Merrick Garland, but will wait until after the next president is in place.

McConnell said the view of the GOP is "give the people a voice in the filling of this vacancy."

Obama Supreme Court

Judge Merrick B. Garland is seen at the federal courthouse in Washington in this May 2008 photo. (Charles Dharapak/Associated Press)

Ahead 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 GOP's most comprehensive judicial response effort ever.

On the other side, Obama allies have been drafted to run a Democratic effort that will involve liberal groups that hope an Obama nominee could pull the high court's ideological balance to the left. The effort would target states where activists believe Republicans will feel political heat for opposing hearings once Obama announced his nominee.

For Obama, Garland represents a significant departure from his past two Supreme Court choices. In nominating Sonia Sotomayor and Elena Kagan, the president eagerly seized the chance to broaden the court's diversity and rebalance the overwhelming male institution. Sotomayor was the first Hispanic confirmed to the court, Kagan only the fourth woman.

Garland — a white, male jurist with an Ivy League pedigree and career spent largely in the upper echelon of Washington's legal elite — breaks no barriers. At 63 years old, he would be the oldest Supreme Court nominee since Lewis Powell, who was 64 when he was confirmed in late 1971.

Presidents tend to appoint young judges with the hope they will shape the court's direction for as long as possible.

Those factors had, until now, made Garland something of a perpetual bridesmaid, repeatedly on Obama's Supreme Court lists, but never chosen.

Democratic presidential hopefuls Hillary Clinton and Bernie Sanders support Obama's choice of nominee, while Republican contenders Ted Cruz and John Kasich stand with those in their party who believe the Senate should not vote on any nominee until after the next president is sworn into office.

With files from CBC News