WASHINGTON — President Barack Obama nominated veteran appellate court judge Merrick Garland to the U.S. Supreme Court on Wednesday, setting up a potentially ferocious political showdown with Senate Republicans who have vowed to block any Obama nominee.

Considered a moderate, Garland, 63, was picked to replace long-serving conservative Justice Antonin Scalia, who died on Feb. 13. A Chicagoan like Obama, he serves as chief judge of the influential U.S. Court of Appeals for the District of Columbia Circuit and is a former prosecutor who in the past has won praise from both Republicans and Democrats.

Wasting no time in pressing its case for Senate confirmation, the administration is dispatching Garland to Capitol Hill on Thursday to huddle with Sen. Patrick Leahy of Vermont, the senior Judiciary Committee Democrat and then with Senate Democratic leader Harry Reid of Nevada.

Such meetings are aimed at shoring up Senate support for the nominee and generating media coverage. The lifetime appointment to the high court requires Senate confirmation.

Obama’s announcement prompted a flood of reaction from private groups that will work to advance or kill the nomination.

The UAW, representing automobile, aerospace and some agricultural workers, call Garland “a distinguished, moderate judge with more federal judicial experience than any other Supreme Court nominee in history.”

National Rifle Association Executive Director Chris Cox said, “A basic analysis of Merrick Garland’s judicial record shows that he does not respect our fundamental, individual right to keep and bear arms for self-defense.”

Speaking in the White House Rose Garden as his administration girded for a fight, Obama urged Senate Republicans to consider the nomination, saying faith in the American justice system was at stake.

“I’ve 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, even-handedness and excellence,” Obama said.

“These qualities and his long commitment to public service have earned him the respect and admiration of leaders from both sides of the aisle [Democrats and Republicans].”

Senate confirmation is required for any nominee to join the bench and Scalia’s sudden death set off an election-year fight well before Obama made his choice.

Republicans, hoping a candidate from their party will win the Nov. 8 presidential election, are demanding that Obama leave the seat vacant and let the next president, to be sworn in next January, make the selection.

Senate Republicans, who control the chamber, have vowed not to hold confirmation hearings or a vote on any nominee picked by the Democratic president for the lifetime position on the court.

Obama said on Wednesday that if Senate Republicans refused to carry out their constitutional function to consider Garland’s nomination, the reputation of the Supreme Court and faith in the American justice system would suffer.

“Our democracy will ultimately suffer as well,” Obama added.

“I have fulfilled my constitutional duty. Now it’s time for the Senate to do theirs. Presidents do not stop working in the final year of their term. Neither should a senator,” added Obama.

He said he hoped the Senate would vote to confirm Garland in time for him to join the court when it gets to work for its 2016-1017 term in October, adding Garland would start meeting with senators one-on-one on Thursday.

One of those senators, who could hold a key to whether Garland’s nomination moves forward, is Maine Sen. Susan Collins, a moderate Republican. Collins said Wednesday that she planned to meet with Garland and treat him as she has past Supreme Court nominees.

“Judge Garland is a capable and accomplished jurist,” Collins said in a prepared statement. “The White House has requested that I meet with him, and I look forward to doing so, as has been my practice with all Supreme Court nominees.”

Collins can expect to face intense pressure from the White House and from conservative groups that have already begun lobbying senators against considering Garland’s nomination before the November election.

Maine’s other U.S. senator, Angus King, an independent who caucuses with Democrats, urged his colleagues to act on Garland’s nomination.

“I intend to carefully review Mr. Garland’s record and evaluate his character, intellect, and judicial temperament, and I hope all of my colleagues — regardless of their politics — will join me by doing the same,” King said Wednesday.

Garland is a longtime appellate judge and former prosecutor who Obama also considered when he filled two previous Supreme Court vacancies.

Standing in between Obama and Vice President Joe Biden during the Rose Garden ceremony, Garland told Obama it was a great privilege to be nominated to the high court by a fellow Chicagoan.

“[A] life of public service is as much a gift to the person who serves as it is to those he is serving. And for me there could be no higher public service than serving as a member of the United States Supreme Court,” said Garland, who would become the fourth Jewish member of the nine-member court.

In a foreshadowing of the pressure campaign the White House and its allies plan to wage in the coming weeks, the White House pointed out that seven current Republican U.S. senators voted to confirm Garland to the DC Circuit court in 1997. The seven are: Collins, Dan Coats of Indiana, Thad Cochran of Mississippi, Orrin Hatch of Utah, James M. Inhofe of Oklahoma, John McCain of Arizona, and Pat Roberts of Kansas.

Garland, who has earned praise from lawmakers of both parties in the past, is no stranger to long, drawn-out confirmation processes. Former President Bill Clinton initially nominated him to the bench in 1995, but his candidacy got mired in a bitter dispute fueled by Charles Grassley, who wanted to reduce the number of seats on the D.C. bench. It would take until March 1997 before the Senate voted to confirm Garland’s appointment, 76 to 23.

Grassley, R-Iowa, is the chairman of the Senate Judiciary Committee and has been firm in stating that his committee will not hold confirmation hearings for Obama’s nominee.

Prior to serving on the U.S. Court of Appeals for the District of Columbia, Garland worked in the Justice Department during the Clinton administration.

Federal appeals court judge Sri Srinivasan had also been a finalist for the nomination.

Without Scalia, the nine-member Supreme Court is evenly split with four liberals and four conservative justices. Obama’s nominee could tilt the court to the left for the first time in decades.

Republicans are hoping to keep the vacancy until the new president takes office, and hoping their candidate wins. Currently, billionaire Donald Trump is the leading Republican presidential candidate while Obama’s former secretary of state, Hillary Clinton, is the front-runner on the Democratic side.

Republicans and their allies already have geared up to fight Obama’s nominee. The Republican National Committee on Monday announced the formation of a task force that will work with an outside conservative group to spearhead attack ads and other ways of pushing back against Obama’s choice.

At least a couple of Republican senators acknowledged discussions of whether the GOP Senate might confirm an Obama nominee in a “lame duck” session after the election, should Clinton be elected president.

“If the election doesn’t go the way Republicans want it, there will be a lot of people open to that I’m sure,” said Sen. Jeff Flake, R-Arizona, who sits on the Judiciary Committee.

Clinton, the front-runner for the Democratic presidential nomination, said Obama “has met his responsibility” under the Constitution and that now it is up to senators to meet theirs.

“Evaluating and confirming a Justice to sit on this nation’s highest court should not be an exercise in political brinkmanship and partisan posturing,” she said in a statement Wednesday. “It is a serious obligation, performed on behalf of the American people, to ensure a highly qualified candidate fills a vacancy on the Court. That obligation does not depend on the party affiliation of a sitting president, nor does the Constitution make an exception to that duty in an election year.”

Clinton hailed the choice of Garland, saying he has “a brilliant legal mind, and a long history of bipartisan support and admiration.” She called on the Senate to take up the nomination “immediately” and said refusing to do so would be “entirely unacceptable.”

If the Senate declines to take up Garland’s nomination before Obama leaves office, or votes it down, the next president will have the option of resurrecting the nomination or choosing someone else to fill the vacancy on the Supreme Court. Either way, the process would begin anew with the next Congress.

White House officials, however, said they were not entertaining such a possibility. “We expect Chief Judge Garland to be confirmed in this Congress, period,” White House spokesman Eric Schultz told reporters in a telephone call Wednesday.

A moderate record

The U.S. Court of Appeals for the District of Columbia Circuit has served as a springboard to the Supreme Court for several justices including Scalia in recent decades.

Obama may have been looking for a nominee who could convince the Republicans to change course. Garland could fit that bill with his moderate record, background as a prosecutor and history of drawing Republican support.

Garland was under consideration by Obama when he filled two prior high court vacancies. Obama, in office since 2009, has already named two justices to the Supreme Court: Sonia Sotomayor, who at 55 became the first Hispanic justice in 2009, and Elena Kagan, who was 50 when she became the fourth woman ever to serve on the court in 2010.

Presidents tend to pick nominees younger than Garland, so they can serve for decades and extend a president’s legacy. But Obama may reason that the choice of an older nominee might also entice Senate Republicans into considering Obama’s selection.

Mixed reaction

Initial reaction from interest groups supportive of the president was mixed. National Organization for Women President Terry O’Neill praised Garland for “a rigorous intellect, impeccable credentials, and a record of excellence.”

But she also said his record on women’s rights was “more or less a blank slate. Equally unfortunate is that we have to continue to wait for the first African-American woman to be named. For this nomination, the so-called political experts ruled that the best choice for the highest court in the nation was a cipher — a real nowhere man.”

A four-page document circulated Tuesday afternoon among a small group of the administration’s allies, with the heading, “Read What Republicans Had to Say About President Obama’s Supreme Court Nominee, Merrick Garland, Before He Was President Obama’s Supreme Court Nominee,” highlighted the support he has enjoyed from lawmakers in the past.

“Garland has had a distinguished legal career, and prior to the GOP’s historically unprecedented obstruction, was a favorite of Senate Republicans alongside progressives,” the briefing material says. “When earlier Supreme Court vacancies occurred in the seats now filled by Justices Elena Kagan and Sonia Sotomayor, Utah Sen. Orrin Hatch said he would be ‘very well supported by all sides’ as a SCOTUS nominee.”

The document notes that when Obama was filling the first Supreme Court vacancy of his tenure, Hatch was quoted at the time as saying that Garland would be a “consensus nominee” who “would be very well supported by all sides.” The briefing material includes previous descriptions of Garland by leading news organizations as a potential nominee who would attract support of Democrats and Republicans alike.

Chief Justice John G. Roberts Jr., Garland’s colleague on the D.C. Circuit, once said that “anytime Judge Garland disagrees, you know you’re in a difficult area.”

An election issue

While the question of who sits on the nation’s highest court is not traditionally a top-tier election issue, Democrats are hoping to use it as part of a broader narrative about Republican resistance to the president’s policies.

David Greenberg, a professor of history and journalism and media studies at Rutgers University, pointed out that Richard Nixon first elevated the Supreme Court as an electoral issue in 1968, when he attacked then-Chief Justice Earl Warren and his fellow justices.

“It was putting a liberal-dominated court at the center of the liberal establishment he was attacking,” Greenberg said, for “bringing about all these cultural changes” in the country.

At the moment, more Americans appear to be sympathetic to the White House’s argument. Sixty-three percent of Americans said the Senate should hold hearings on Obama’s nominee to replace Scalia, while 32 percent said it should not hold hearings and leave it to the next president, according to a Washington Post-ABC News poll released last week. Majorities of Democrats and independents supported holding hearings, while Republicans were more evenly split (46-49) and over half of conservative Republicans said hearings should not be held (54 percent).

Administration officials are hopeful that the GOP senators who are most vulnerable this November — Sens. Kelly Ayotte of New Hampshire, Ron Johnson of Wisconsin, Mark Kirk of Illinois and Pat Toomey of Pennsylvania — may lobby their leaders for a vote if they come under fire back home for blocking the nominee.

“The success or failure of this will depend on the pressure that can be brought to bear on those senators who Mitch McConnell marched out to the firing line,” said one former senior administration official.

Contributing to this story were Washington Post reporters Juliet Eilperin, Mike DeBonis, Jerry Markon, William Branigin, Mark Berman and Robert Barnes; and Reuters reporters Jeff Mason and Richard Cowan.