WASHINGTON — Hundreds of progressive activists crowded into the Hamilton music venue a few blocks from the White House on Monday night to celebrate the nomination of Judge Ketanji Brown Jackson, listen to queen of funk Chaka Khan perform and hear someone, as the song goes, tell them something good.

Senator Elizabeth Warren, Democrat of Massachusetts, delivered.

“Judge Ketanji Brown Jackson is going to make a fabulous justice,” Ms. Warren told the crowd filled with Black public defenders, who cheered the prospect that someone who looked like them and shared their professional background appeared headed to the high court for the first time.

The “Have Her Back” concert was organized by Demand Justice, the progressive judicial advocacy group that has become a favorite target of attack for Republicans as they line up against President Biden’s Supreme Court nominee.

Republicans have assailed the “dark money” organization — its donors are undisclosed, like those of similar judicial advocacy groups on the right — for its calls to expand the Supreme Court in order to offset its conservative slant. Progressives contend that Republicans, through hardball tactics, effectively stole two seats on the high court, and adding justices is the only way to rebalance it.

Mr. Biden has not endorsed the move, and the commission he named to study it split on the issue and declined to weigh in. Judge Jackson has also studiously avoided giving her opinion. But Republicans have cited Demand Justice’s fervent support for Judge Jackson as evidence that she embraces the push. Her refusal during Senate Judiciary Committee hearings last week to reject the idea of adding seats to the court — she said it was a policy decision, not a judicial one — has only hardened those suspicions.

Senator Mitch McConnell, Republican of Kentucky and the minority leader, regularly criticizes Judge Jackson’s support from progressives. In saying he would oppose her nomination, Senator Thom Tillis, Republican of North Carolina, said that he was “disappointed that she is reluctant to take a firm stand against a liberal, dark-money court-packing scheme.”

“Birds of a feather,” said Senator John Cornyn of Texas, one of many Republicans on the Judiciary Committee who pressed Judge Jackson on why Demand Justice was supporting her, even as she testified that she had no relationship with the group. “I think a fair question is, if this group is so determined to see you confirmed, do you agree with their agenda?”

Democrats brushed off the idea that Judge Jackson is the Demand Justice nominee.

“To suggest that you’re here merely because an organization supports you ignores your qualifications and the broad range of support you bring to this,” Senator Richard J. Durbin, Democrat of Illinois and the chairman of the panel, told her last month as the hearing opened.

The attention has been a double-edged sword for Demand Justice. It has raised the group’s profile and its ability to raise money, making March the group’s best fund-raising month ever — more than 50 percent ahead of the second-best — according to one person with knowledge of its finances. But its work on Judge Jackson’s behalf, part of about $12 million the organization is expected to spend this year, has complicated her effort to win confirmation by giving Republicans a ready line of attack and a justification to oppose her.

Brian Fallon, a former top Democratic operative and co-founder of Demand Justice, said some Republicans decided it would be better to go after the group rather than the judge herself.

“The right made a decision early on that they were unlikely to defeat Biden’s nominee since Democrats controlled the Senate and it would be politically dicey to too forcefully oppose the first Black woman anyway,” he said.

Mr. Fallon and other progressives from Capitol Hill and the White House started Demand Justice in 2018, two years after Republicans effectively kept a Supreme Court seat open for President Donald J. Trump by refusing to even grant a hearing for Merrick B. Garland, President Barack Obama’s third Supreme Court nominee, citing the fact that it was a presidential election year. Organizers wanted to instill the same passion about the courts on the left as is usually found on the right.

The group began clamoring to add seats to the court and stepped up their calls after Republicans rushed through the confirmation of Mr. Trump’s third nominee, Judge Amy Coney Barrett, just days before he lost the 2020 election.

While the Federalist Society, a conservative group focused on the judiciary, is closely aligned with Republicans, Demand Justice has applied some of its pressure to Democrats, urging them to get more aggressive when it comes to filling judicial vacancies. Their efforts have cost the organization some friends, including Mr. Durbin, after they pushed last year for someone else to be chairman of the Judiciary Committee.

They have also used their own version of hardball tactics to try to accomplish their goals. Demand Justice helped instigate an extraordinary public campaign to pressure Justice Stephen G. Breyer to retire while Democrats held the Senate and the White House, to ensure that the party would control the process of replacing him. Justice Breyer, 83, ultimately announced in January that he would leave at the end of the court’s current term, opening the door to Judge Jackson’s nomination.

When Mr. Biden took office, he chose Paige Herwig, a former Demand Justice lawyer, to help screen his judicial candidates, and the group intensified its campaign to diversify the professional background of federal judges, calling for more public defenders and civil rights lawyers in the mix of nominees, rather than the prosecutors and corporate attorneys that usually dominate.

The Biden administration has embraced that approach, with nearly 30 percent of its nominees having experience as public defenders. Judge Jackson is the most prominent example. Demand Justice notes that is a stark change from even the Obama administration, which nominated three prosecutors for every criminal defense lawyer.

“Public defenders need to be treated as the champion for constitutional rights that they are,” Tamara Brummer, a senior adviser for engagement and outreach at Demand Justice, declared at the concert on Monday, calling Mr. Biden “the public defender president.”

The event, which one organizer facetiously labeled a Dark Money Dance Party, was put together by Demand Justice along with Black Lives Matter, the National Legal Aid & Defender Association, the National Association of Criminal Defense Lawyers, MoveOn and the United State of Women. It served not only as an affirmation of public defenders but a chance to hit Republicans for their combative questioning of Judge Jackson in her hearings.

“Instead of having an adult conversation like you might expect at a hearing for the confirmation of a Supreme Court justice, they decide instead they are going to destroy her record,” said Ms. Warren, who was among the first to call for more public defenders on the federal bench in a 2013 speech. “They decide instead that they’re going to shout and scream and talk over her. They decide instead they are going to treat her with disrespect.”

“They can kick and scream,” she added, “but it is not going to change the reality that we are going to confirm this woman to the United States Supreme Court.”

Demand Justice is not stopping with Judge Jackson. The organization has been recruiting potential judicial candidates around the country, conferring with lawyers with diverse professional backgrounds, and conducting online tutorials and interview training. The group says that 18 Biden nominees have disclosed in their paperwork that they have been in contact with Demand Justice.

While the group has become a top Republican target, Mr. Fallon said it had a long way to go to reach parity with Republican-allied organizations with deeper pockets and more history — not to mention dark money sources of their own.

“We don’t mind the attention, because after years of seeing Democrats take it on the chin, we are trying to provide a counterweight to these far-right groups,” he said. “But it’s far-fetched to pretend this is close to a fair fight.”