Washington (CNN) President Barack Obama nominated Judge Merrick Garland , who is respected across political lines, to the Supreme Court Wednesday, in an epic power play targeting the resolve of Republicans who have vowed to block any replacement for the late Justice Antonin Scalia until a new president takes office.

GOP leaders, caught in the undertow of an election in which the conservative grass-roots are already in revolt, immediately renewed their refusal to consider Garland, 63, saying their reservations were not personal but motivated by a desire for the American people to weigh in on Scalia's replacement. The showdown is even more fraught than most Supreme Court fights, since Obama's choice could tilt the ideological balance of the court away from conservatives -- possibly for years.

In a speech in the evocative ceremonial surroundings of the White House Rose Garden, Obama praised Garland as "one of America's sharpest legal minds," making a case that he was so eminently qualified for the job in terms of legal learning, experience and temperament that any attempt to ignore his appointment could only be the result of base political motivations.

"I have selected a nominee who is widely recognized not only as one of America's sharpest legal minds, but someone who brings to his work a spirit of decency modesty, integrity, evenhandedness and excellence," Obama said. These qualities and his long commitment to public service have earned him the respect and admiration from leaders from both sides of the aisle."

Senate Republicans do not plan to vet or have hearings on Garland, let alone a vote on his nomination. Obama and Democrats argue that with 10 months left in his term, there is plenty of time for the Senate to take up and confirm a new justice. The gravity of Obama's announcement on Wednesday was part of an attempt to pressure Republicans, especially senators with an eye on their own legacies or those who face tough re-election fights, to peel away from their leadership.

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But Senate Majority Leader Mitch McConnell and the GOP chairman of the Senate Judiciary Committee, Chuck Grassley of Iowa, have both expressed little leeway in their determination to forgo hearings for Obama's nominee.

"The American people may well elect a president who decides to nominate Judge Garland for Senate consideration," McConnell said Wednesday. "The next president may also nominate someone very different. Either way, our view is this: Give the people a voice in the filling of this vacancy."

McConnell spoke with Garland this afternoon by phone.

"Rather than put Judge Garland through more unnecessary political routines orchestrated by the White House, the leader decided it would be more considerate of the nominee's time to speak with him today by phone," McConnell spokesman Don Stewart said. "And since the Senate will not be acting on this nomination, he would not be holding a perfunctory meeting, but he wished Judge Garland well."

'Greatest honor of my life'

Garland, the chief judge for the U.S. Court of Appeals for the District of Columbia Circuit, has been on short lists before. An appointee of President Bill Clinton, Garland is a graduate of Harvard and Harvard Law School. As a Justice Department lawyer, he supervised investigations in the Unabomber case as well as the Oklahoma City bombing.

His voice cracking with emotion, Garland called Obama's decision to nominate him "the greatest honor of my life."

"For me, their could be no higher public service than serving as a member of the Supreme Court," Garland said.

Obama's announcement amplifies the ongoing political battle over the precedent and propriety of considering a Supreme Court nomination amid a heated presidential election.

The announcement comes after a big night in the 2016 election, with both party's front-runners -- Democrat Hillary Clinton and Republican Donald Trump -- emerging with sweeping victories as they march toward their respective nominations. Some believed Obama would time his pick so it wouldn't get lost in a news cycle dominated by election results. But the timing seems suited to directly insert the selection into the political conversation.

Fueling the argument is the potential for the first shift in the court's ideological leaning in two decades. If confirmed, Obama's nominee will likely offer a vastly different legal outlook that Scalia, who was considered one of the court's most conservative members.

CNN senior legal analyst Jeffrey Toobin said the pick of Garland, who is well-respected in legal circles "takes the issue of qualifications off the table," but noted the fight now is all about politics."

"He is the chief judge of the second most important court in the country, but the political calculation here is, I think, quite clear," Toobin said. "President Obama is saying to the Senate Republicans, you can take my 63-year-old now or wait for President Hillary Clinton to bring up a 45-year-old in 10 months."

At 63, Garland is much older than the other contenders on the short list such as Judges Sri Srinivasan, Paul Watford and Jane Kelly. Garland's supporters argue he is the nominee that the senators couldn't refuse even in a contentious environment.

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"It is my intention to nominate somebody who has impeccable credentials, somebody who should be a consensus candidate," Obama told CNN en Español anchor Juan Carlos Lopez in an interview last week.

This is Obama's third nomination to the high court. Sonia Sotomayor and Elena Kagan were confirmed in 2009 and 2010, respectively. But those confirmation hearings and votes occurred when Democrats were firmly in control of the Senate.

All eyes on Senate Republicans

Democrats have already begun a campaign to pressure Republicans into considering Obama's nomination. In their sights: vulnerable senators up for reelection, some of whom are already facing backlash from opponents for refusing to consider even a hypothetical Obama nominee.

The White House launched a Twitter account, @SCOTUSnom , designed to promote the nomination as well.

But while some moderate Republicans have expressed willingness to meet with the nominee, their ability to force hearings before the full judiciary panel appear slim.

A McConnell aide said noted the Kentucky senator voted against Garland when he was confirmed to the appeals court in 1997 and argued Obama's choice is a recognition that this pick not getting through -- otherwise the President would have picked a younger nominee who would serve longer.

Sen. Pat Roberts, one of seven current Republicans to vote for Garland in 1997, told CNN he will not back the judge now.

"It's not about the person. It's about the process. Let the people decide," Roberts said.

Asked why he voted for Garland previously, Roberts replied, "I supported him."

Trump said he agrees with the stance of the Hill Republicans. "I think the next president should make the pick. And I think they shouldn't go forward. And I believe I'm pretty much in line with what the Republicans are saying," Trump told CNN's Chris Cuomo Wednesday on "New Day."

Toobin said it will be difficult for them to change tactics now.

"I don't see how at this point they can go back on this promise," not to move on the nominee, Toobin said Wednesday morning.

Polls show most Americans support giving a nominee a congressional hearing. A CNN/ORC survey taken late last month indicated sizable majorities of Republicans, Democrats and independents want Senate Republican leaders to hold hearings on the nominee.

Grassley last week said the Senate retained a prerogative to forgo hearings for Obama's selection.

"It isn't any different if the President of the United States notifies Congress well in advance of a piece of legislation that he's going to veto it," Grassley said at a Judiciary panel hearing, citing criticism from Republicans over the role of the high court.

"Whether it's today or tomorrow or whether it's for the next seven or eight months, this is a very important debate that we ought to have about the Constitution and about not only who's going to be a replacement for Justice Scalia but about the role of the Supreme Court," he added. "At the grassroots of America, there's a real feeling of 'Is the Supreme Court doing what the Constitution requires?'"

The announcement comes 32 days after Scalia's death, only slightly longer than it took him to name his two previous appointments to the high court. Unlike his nominations of Sotomayor and Kagan, the vacancy this time wasn't expected. White House officials have said they weren't anticipating another Supreme Court nomination during Obama's term ahead of Scalia's death.

Obama oversaw a team led by his counsel Neil Eggleston, chief of staff Denis McDonough, and his senior adviser Brian Deese to select and vet a group of potential nominees. After conducting interviews last week, Obama narrowed his list to include Merrick, Watford and Srinivasan, each of them considered "consensus" candidates for their history in gaining confirmation support from Republicans.

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Any replacement of Scalia has the power to tilt the ideological balance of the court for decades, something conservatives are using to move their base to hold the line.

"This seat could be transformational to the court because Justice Scalia's fidelity to the Constitution was a real anchor for the court. If he were replaced by an Obama nominee that would give the court a solid five votes for enacting an extremely liberal agenda that the American people will not be comfortable with," Carrie Severino, of Judicial Crisis Network , a conservative group opposed to any candidate getting a hearing until after the election. It would shift the court --that is somewhat balanced --to a liberal stronghold. She is a former clerk of Justice Clarence Thomas.

Since Scalia's death and for the foreseeable future, the court has operated with eight justices, four appointed by Democrats and four by Republicans.

Justices have been considering several major cases, including a challenge to public sector unions, a race-conscious admissions plan at the University of Texas, the first big abortion case since 2007, challenges to voting rights, the Affordable Care Act's contraceptive mandate and a challenge to Obama's executive actions on immigration. Scalia's death means not only the loss of the court's main conservative voice but also increases the likelihood of a 4-4 split on controversial issues. If the court is equally divided in a case, ruling 4-4, it means the lower court opinion stands and there is no precedent set by the Supreme Court.