Seated from left are Justices Stephen G. Breyer and Clarence Thomas and Chief Justice John G. Roberts Jr. Standing from left are Justices Neil M. Gorsuch, Sonia Sotomayor and Elena Kagan. (Jabin Botsford/The Washington Post)

The once-remote idea of adding more justices to the Supreme Court to change its ideological bent is prompting growing discussion within the Democratic Party, creating a new frontier for presidential candidates looking to display their liberal credentials.

Former attorney general Eric H. Holder Jr., who recently decided against running for president, became the latest figure to embrace an expansion of the nine-member court in recent talks at Yale Law School and Columbia University.

He questioned the validity of the current court, given Senate Republicans’ refusal to vote on Judge Merrick Garland after President Barack Obama nominated him to the Supreme Court in March 2016. The seat was ultimately filled by President Trump’s nomination of Justice Neil M. Gorsuch.

“Given the Merrick Garland situation, the question of legitimacy is one that I think we should talk about,” Holder said. “We should be talking even about expanding the number of people who serve on the Supreme Court, if there is a Democratic president and a Congress that would do that.”

His comments come as activists launch an organized effort to prod the presidential contenders to say publicly they’re open to such ideas. Sen. Kirsten Gillibrand (D-N.Y.) has called adding justices — or imposing term limits on them — “interesting ideas that I would have to think more about.”

Pete Buttigieg, the mayor of South Bend, Ind., who is exploring a presidential campaign, has responded to questions by inviting consideration of either adding justices or rotating them on and off the court. He has also discussed a 15-justice structure for the court — five Democratic appointees, five Republican appointees and five chosen by the other 10.

Conservatives have long been galvanized by the promise of moving the Supreme Court to the right, making it a sometimes significant factor in their voting decisions. Now liberals, angered by the aggressive GOP push to remake the federal courts, are becoming equally impassioned, prompting the discussion of far-reaching ideas for remaking the nation’s highest legal body.

The ideas face an uphill path to becoming law, given that resistance from Republicans is likely. But the discussion itself reflects Democrats’ frustration in the Trump era.

The concept of expanding the Supreme Court, like the phrase “court packing” itself, fell into lengthy disrepute after 1937, when President Franklin D. Roosevelt — frustrated that conservative justices were blocking New Deal programs he considered to be crucial for the country — sought to add six friendlier justices, prompting an outcry even from allies.

Some liberal activists and scholars say the idea now merits a new look. They view the GOP’s refusal to consider Garland as tantamount to stealing a Supreme Court seat, and they contend that the Senate paid insufficient attention to the sexual misconduct allegations against Justice Brett M. Kavanaugh in last year’s confirmation hearing, which he forcefully denied.

Republicans, these activists fear, have in essence created a judicial buffer against any future liberal agenda if Democrats recapture the White House and Congress.

“Democrats cannot sit back and accept the status quo of a partisan Republican five-seat majority for the next 30 years,” said Brian Fallon, a former adviser to presidential candidate Hillary Clinton and to Senate Minority Leader Charles E. Schumer (D-N.Y.) who now runs the advocacy group Demand Justice. “We don’t consider those two seats that Trump has filled to be legitimate.”

Fallon has been in contact with a new group, Pack the Courts, which seeks to inject the idea of restructuring the court into the presidential campaign. Supporters of the group were responsible for publicizing Holder’s recent comments and for prompting Buttigieg’s response, and they have reached out to other campaigns.

“The strategy is to make the 2020 candidates understand that if they don’t come up with an agenda to deal with the courts, everything they are talking about is going to be dead on arrival,” said Aaron Belkin, executive director of Pack the Courts and a professor at San Francisco State University.

Leaders of the grass-roots network Indivisible, which helped organize for Democrats in the 2018 campaign, have also voiced support for changes in the court. Ezra Levin, the group’s co-executive director, said the idea comes up regularly in the group’s meetings.

Activists such as Levin say that Senate Majority Leader Mitch McConnell (R-Ky.) has warped the judicial nomination process by using unprecedented tactics to confirm young, far-right judges.

“Indivisible is strongly against packing the courts — which is why we think we need to expand the courts, to undo all the packing that has been done in recent years,” Levin said.

McConnell, for his part, says Democrats are merely upset at Republican success in putting qualified conservative judges on the bench despite Democratic obstructionism. The new interest in court packing, he says, reflects liberal extremism.

“Some left-wing publications are already trying to lay the groundwork for, you guessed it, literally packing the court with more justices,” McConnell said on the Senate floor after Kavanaugh’s swearing-in ceremony. “That’s right, the far left has gone scrounging through the ash heap of American history, and they’re bandying about that discredited fantasy from the 1930s.”

Members of both parties, however, decry the fact that Supreme Court confirmation fights, including Kavanaugh’s, have increasingly degenerated into drawn-out battles featuring personal attacks, bitter recriminations and charges of bad faith.

Rep. Ro Khanna (D-Calif.) has endorsed a proposal from Bruce Ackerman, a professor at Yale Law School, to end lifetime appointment to the Supreme Court and instead shift justices onto lower courts after stints of 14 or 18 years. Under this plan, presidents would appoint justices at regular intervals, arguably lowering the temperature and the stakes for each confirmation.

“There is something that is wrong in our country when our Supreme Court hearings are becoming the high-stakes affairs that they are, almost like presidential elections,” Khanna said. “The Founding Fathers never intended that.”

Legal scholars say it’s not clear whether the Constitution allows term limits for justices if they remain on the federal bench after leaving the high court. The Constitution says federal judges “both of the supreme and inferior Courts, shall hold their Offices during good Behaviour,” which has generally been interpreted to mean lifetime appointments.

The Constitution is also silent on the size of the Supreme Court. There were six justices after the Judiciary Act of 1789 and 10 under President Abraham Lincoln, and there have been nine since 1869.

The current debate comes as the Supreme Court, and the judiciary more broadly, is increasingly a focus of impassioned political debate. Both major-party presidential candidates in 2016 outlined explicit litmus tests for their nominees. Trump has repeatedly attacked federal judges as political actors when they rule against his interests, sometimes calling them “Obama judges.”

That prompted Chief Justice John G. Roberts Jr., a defender of the judiciary who was appointed by President George W. Bush, to take the rare step of publicly pushing back.

“We do not have Obama judges or Trump judges, Bush judges or Clinton judges,” Roberts said in November. “What we have is an extraordinary group of dedicated judges doing their level best to do equal right to those appearing before them.”

For some proponents of court packing, the current effort could bring benefits even if Congress never votes on it. Activists say the threat alone could pressure the justices to avoid decisions that would inflame partisan fury by undoing legislative or presidential actions.

“If Roberts is genuinely looking to preserve his institution, he has some runway here,” said Ian Millhiser, a columnist at the liberal website ThinkProgress who wrote a recent article defending court packing.

Millhiser and others say they’re particularly concerned about potential court decisions on gerrymandering, voting rights and other matters that directly affect the democratic process and could make it harder for Democrats to win elections.

“It’s the existential democratic issues that provide the most principled basis” for a change to the Supreme Court, said Todd Tucker, a political scientist at the Roosevelt Institute.

Some legal scholars warn, though, that the debate over court packing risks further weakening the court’s credibility because it suggests that the court is irrevocably political in its current form.

“The risk is really, one way or another, undermining the ability of the court to play the role it has played in American democracy,” said David Strauss, a professor at the University of Chicago Law School. “People have to be willing to go along with what the court does.”

Roberts has made clear over the years that he is sensitive to the danger of politicization. In a 2015 address to the Historical Society of the New York Courts, he praised former chief justice Charles Evans Hughes, who successfully worked to defeat Roosevelt’s plan to add six justices to the court.

“It fell to Hughes to guide a very unpopular Supreme Court through that high-noon showdown against America’s most popular president since George Washington,” Roberts said.

He also praised Hughes for working “under the radar” to allow time for Congress to realize the harm that would come from Roosevelt’s proposal.

Robert Barnes contributed to this report.