WASHINGTON (Reuters) - As President Barack Obama unveiled his Supreme Court nominee on Wednesday, a carefully crafted operation to sell his choice to Americans and to resistant Republicans rolled into action.

President Barack Obama smiles after announcing Judge Merrick Garland (L) of the United States Court of Appeals as his nominee for the U.S. Supreme Court in the Rose Garden of the White House in Washington March 16, 2016. REUTERS/Kevin Lamarque

Liberal groups announced a national campaign to target Republican Senate opponents with demonstrations and television ads in what is likely to be one of the most bitter fights ever over a Supreme Court nomination.

The White House has enlisted legal scholars and corporate executives to make the case for centrist judge Merrick Garland despite resistance from Senate Republican Leader Mitch McConnell and other senior Republicans who have said Obama should let the next president fill the position.

Garland himself, an appeals-court judge, immediately started making calls to Capitol Hill, signaling the start of a battle for public perceptions as Obama seeks to highlight what Democrats characterize as unprecedented obstructionism.

The pressure may not be enough to convince Republicans to give Garland a hearing, let alone a confirmation vote.

But the White House is betting that since many Republicans face tough re-election battles, they may be less willing to reject Garland out of hand.

The White House has enlisted former White House officials Stephanie Cutter and Katie Beirne Fallon to mobilize liberal groups, which see the fight as a historic chance to push the Supreme Court to the left as well as mobilize their supporters ahead of the elections.

“We’re going to organize in record numbers,” said Mary Kay Henry, president of the Service Employees International Union, which plans rallies outside the offices of Republican senators in Pennsylvania, Ohio, Illinois, Wisconsin and New Hampshire starting next week.

MOVING THE NEEDLE

Republicans likely won’t be swayed by such messages, but voters may be, said Michael Gottlieb, a lawyer at Washington firm Boies, Schiller, & Flexner who was involved with the last four Supreme Court nominations.

“What moves the needle is if your average person, an independent voter or a moderate Republican, sees inaction on a nominee as the Senate not doing its job,” Gottlieb said.

Some 57 percent of Americans believe Obama should name a replacement for the vacant seat while 33 percent say he should not, according to Reuters/Ipsos polling.

Events planned by a slew of Democrat-linked groups will feature “working Americans telling Senate Republicans to do their jobs,” according to a former White House official who is involved in the efforts.

Social media will play a big part. A petition under the Twitter tag #DoYouJob has already collected 1.5 million signatures.

A central target is Republican Senator Charles Grassley of Iowa, who has said any Obama nominee would not get a hearing before his Judiciary Committee.

Facing a tough Democratic challenger and pressure from liberal activists, Grassley talked with Garland by phone on Wednesday and held out the possibility of a meeting.

Democratic aides say they aim to pressure Grassley into holding a hearing. Whether he does or not, they see Grassley in the meantime struggling to justify his current hard-line position, which they think could help his Democratic opponent in November, former Lieutenant Governor Patty Judge.

RE-ELECTION WORRIES

Republican senators facing re-election in New Hampshire, Illinois, and Ohio have said they are willing to meet with Garland.

Democrats view those seats as among the most promising targets in their effort to win back control of the 100-seat body. Some 34 seats are up for grabs in the Senate, where Republicans have a 54-46 seat majority.

Democrats say the fight could make Senate races in Missouri and North Carolina competitive as well, though independent analysts say they still face an uphill climb.

With the election still seven months away, “it will be tough to keep the issue at the forefront of voters’ minds,” said Nathan Gonzales, who tracks elections for the Rothenberg & Gonzales Political Report.

Despite the political overtones, Obama opted to pick a relatively uncontroversial candidate. Garland, the top judge on the U.S. Court of Appeals for the District of Columbia, is viewed as a centrist who is not ideologically driven.

Some conservative groups say Garland’s past rulings indicate that he would push the court to the left on gun rights, government regulations and other key issues. But others plan to keep the focus on Obama, not his nominee.

“If you start eviscerating him in the public sphere, you’re going to create sympathy and the White House will use that,” said Jason Pye, a spokesman for libertarian group FreedomWorks. Members of the network have sent more than 500,000 emails urging Republican senators to stick to their position, he said.

The White House said it had enlisted 356 legal scholars, 217 corporate lawyers and 17 environmental executives in a letter-writing campaign urging Republican senators to act.

Some on the left say they may have a hard time rallying their members behind a centrist, 63-year-old white male candidate.

“We have to go where grassroots energy is. We can’t just manufacture it out of thin air,” said Neil Sroka, a spokesman for Democracy For Action, a network of 1 million progressive activists.