Justices of the US Supreme Court pose for their official photo at the Supreme Court in Washington, DC on November 30, 2018. - Standing from left: Associate Justice Neil Gorsuch, Associate Justice Sonia Sotomayor, Associate Justice Elena Kagan and Associate Justice Brett Kavanaugh.Seated from left to right, bottom row: Associate Justice Stephen Breyer, Associate Justice Clarence Thomas, Chief Justice John  Roberts, Associate Justice Ruth Bader Ginsburg and Associate Justice Samuel Alito. (Photo by MANDEL NGAN / AFP)        (Photo credit should read MANDEL NGAN/AFP/Getty Images)
Justices of the US Supreme Court pose for their official photo at the Supreme Court in Washington, DC on November 30, 2018. - Standing from left: Associate Justice Neil Gorsuch, Associate Justice Sonia Sotomayor, Associate Justice Elena Kagan and Associate Justice Brett Kavanaugh.Seated from left to right, bottom row: Associate Justice Stephen Breyer, Associate Justice Clarence Thomas, Chief Justice John Roberts, Associate Justice Ruth Bader Ginsburg and Associate Justice Samuel Alito. (Photo by MANDEL NGAN / AFP) (Photo credit should read MANDEL NGAN/AFP/Getty Images)
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(CNN) —  

Republicans have been successfully leveraging the Supreme Court balance of power as a major campaign issue to ignite their base since the 1980s. For Democrats, the 2020 election may mark the first in modern times that they unite around the high court as a driving force in a presidential election.

Democratic candidates are increasingly advocating “court packing,” that is, upping the number of Supreme Court justices to balance the bench – or ensure a liberal majority. The idea is unlikely to succeed for historical and practical reasons but its resonance on the campaign trail reflects Democrats’ new emphasis on the judiciary during the Trump era.

The issue emerges partly from lingering anger over the Republican-led Senate’s 2016 stall of President Barack Obama’s nominee Merrick Garland, a fire stoked this week when Senate Majority Leader Mitch McConnell said if a Supreme Court vacancy occurred in 2020 election year, he would confirm a Trump nominee.

McConnell’s comments were an audacious reversal of his 2016 election-year position blocking Garland for the vacancy caused by the sudden death that February of Justice Antonin Scalia. McConnell argued that the seat should be filled by whoever won the then-upcoming presidential election.

Also drawing Democratic attention to who sits in the nine black leather chairs at the high court has been the recent enactment in states of new abortion bans. The fate of those restrictions would eventually be decided by the Supreme Court, now featuring four liberals and, with Brett Kavanaugh replacing the retired Anthony Kennedy last year, five solid conservatives.

“Everyone’s been worried about ‘Will the Supreme Court overturn Roe v. Wade,’” California Democratic Sen. Kamala Harris told a Los Angeles audience earlier this month.

Harris said she was open to adding more justices. “I think it’s a conversation that we need to have. I am open to increasing the numbers on the Supreme Court,” she said, responding to a woman who asked about enlarging the court and prefaced her question with a complaint that McConnell “stole a Supreme Court seat.”

“The most critical issues of our lifetimes, before and in the future,” Harris said, “will be decided by that United States Supreme Court.”

In a similar vein, Rep. Hakeem Jeffries, who leads the House Democratic Caucus, said Wednesday on CNN’s “New Day,” “We think that the Supreme Court issue should be a voting issue in November of 2020.” Jeffries was responding to McConnell’s latest remarks about possibly confirming a third Trump justice to the high court.

To be sure, the leading Democratic candidates are not of a single mind on Supreme Court proposals.

Sen. Bernie Sanders of Vermont, for example, has criticized “court packing,” warning that it could end up benefitting Republicans, and he instead has touted term limits for justices.

Both parties have put the Supreme Court and character of the federal judiciary at the center of today’s politics.

President Donald Trump has tweeted that the country needs “more Republicans …. And must ALWAYS hold the Supreme Court,” and, again this month, he disparaged a judge based on his appointment by the Democrat Obama.

Of a ruling that an accounting firm had to turn over Trump financial records, the President said, “It’s totally the wrong decision by, obviously, an Obama-appointed judge.”

Less prominent but equally significant, the Trump administration, partnering with McConnell, has been setting records for the number of life-tenured US appeals court judges appointed. That’s the powerful level just below the Supreme Court.

Trump has won Senate confirmation for 41 appeals court judges, more than twice the number that Obama saw confirmed by this point in his first term. And the Trump pace could continue. The Republican majority has lifted Senate procedural rules and left Democratic senators with scant opportunity to slow the consideration of nominees they oppose.

While these developments elevate the 2020 election-season stakes for the judiciary, they also could come at the cost of judicial independence in the public eye.

“The more you politicize the judiciary,” said New York University law professor Barry Friedman, who has tracked public attitudes toward the Supreme Court, “the more people will question the institution of judicial review.”

Friedman is not surprised by the current Democratic apprehension about the US courts. “It’s cumulative, and they feel something is slipping away,” he said. “The President and Senate are also waving through lower court appointments like they’re at the Grand Prix.”

Chief Justice John Roberts addressed some of the politicization last November, when he responded to another Trump “Obama judge” comment and issued a statement scolding the President and implicitly warning people on both sides: “We do not have Obama judges or Trump judges, Bush judges or Clinton judges. What we have is an extraordinary group of dedicated judges doing their level best to do equal right to those appearing before them.”

Trump has also scoffed at the various Democratic proposals, saying in response to a reporter’s question during a March news conference, “The only reason that they’re doing that is they want to try to catch up. So if they can’t catch up through the ballot box by winning an election, they want to try doing it in a different way. … It’ll never happen. It won’t happen. I guarantee it won’t happen for six years.”

Hot, but longshot, idea

South Bend, Indiana, Mayor Pete Buttigieg was among the first of the Democratic presidential candidates to spotlight court overhaul proposals. He endorsed the possibility of a 15-justice Supreme Court and term limits (rather than life tenure) at a CNN town hall in March.

“What we need to do is stop the Supreme Court from sliding toward being viewed as a nakedly political institution,” he said at the Austin, Texas, forum. “I’m for us contemplating whatever policy options will allow that to be possible.”

By law, Congress can set the number of justices, and it has exercised that power over the decades from a high of 10 justices to a low of five. It has been 150 years since the last change, when Congress established the current nine-member bench.

Any term limits on the justices, however, would require a change to the US Constitution, which established life tenure for federal judges.

President Franklin Delano Roosevelt tried to persuade Congress to add more justices to the Supreme Court in the 1930s after it invalidated several of his New Deal economic initiatives. Roosevelt proposed the appointment of a new justice for every sitting justice older than 70; six of the nine were then over 70. The idea floundered with the public and Congress.

So “court packing” would unsettle 150 years of nine justices and recall FDR’s infamous failure. It also seems impractical even for progressive Democrats promoting the idea. To change the size of the Supreme Court under such plans, Democrats would have to win the White House and the Senate in 2020, as well as hold the current majority in the House.

It they were to succeed, would the party use its new political capital to restructure the high court, rather than turn to economic, environmental and other policy initiatives that might be more appealing to the electorate?

“I would bet no,” says Duke University law professor Neil Siegel, who has worked as a special counsel to Democratic senators during Supreme Court nominations and is currently cautioning liberals against court-packing proposals. Siegel argues that they would invite further politicization of the nation’s courts.

Yet, he adds that the momentum behind court-overhaul ideas at campaign venues reflects people’s sense “that something has gone fundamentally awry … that the court is substantially more conservative than the country.”

Such sentiment is far from bipartisan, of course. Florida Sen. Marco Rubio and a group of Republican colleagues have offered a measure for a constitutional amendment that would keep high court at nine. (Any amendment to the Constitution, whether to lock in the number of justices or to institute term limits, would require ratification by three-fourths of the 50 states: 38.)

There has been little public polling on these ideas. One survey question, however, conducted by Fox News in April, found that when asked about “increasing the number of justices on the US Supreme Court,” a majority of registered voters were against the idea.

Pollsters found 37% of voters were in favor, 51% opposed and 12% unsure. They also found Democrats were more in favor of increasing the number of seats (51%); only 23% of the Republicans polled said they wanted more justices.

The emerging Democratic interest is seen in voters who turn out at candidate forums. At a recent Harris rally at Los Angeles Southwest College, Michele Eason said she had researched scenarios that would allow a future president to add new justices to the Supreme Court.

“I wouldn’t be opposed to the FDR solution to the problem we’re talking about – which was that there’s nothing in the Constitution that says the Supreme Court has to have nine members,” Eason said, adding, “Normally I would say, hey that’s kind of dirty pool and it’s not playing fair. Two words: Merrick Garland.”

CNN’s Jennifer Agiesta and Maeve Reston contributed to this report.