Washington (CNN) Liberal groups are holding standing-room-only meetings to prepare for the confirmation hearings of Sen. Jeff Sessions for attorney general, but many in the room believe the battle will be a dry run for a much bigger fight: the confirmation of Donald Trump's eventual Supreme Court nominee.

The hearings to fill the ninth seat on the bench will center around the testimony of someone who will likely cement a conservative majority on the high court for years to come.

The groups are now working on an unfamiliar playing field -- without the anchor of a Democratic President. They are laying the groundwork for a major fight and hoping that the new ranking member of the Judiciary Committee -- California Sen. Dianne Feinstein -- will lead that fight.

"We are not going to settle on a Supreme Court nominee," Senate Minority Leader Chuck Schumer told MSNBC on Tuesday night. "If they don't appoint someone who's really good, we're going to oppose him tooth and nail."

Democrats are also preparing to put Republicans on trial for the way they handled President Barack Obama's nominee, Merrick Garland.

Liberal groups believe that when Republicans blocked hearings for Garland, they set a new precedent forever changing Supreme Court confirmation hearings. And they refer to the seat that Trump is about to fill as the "stolen seat."

On Tuesday, White House spokesman Josh Earnest warned that the fallout from the Garland fight would extend to future presidents.

"That breakdown of comity in the United States Senate, that abdication of the basic responsibility of members of the Unites States Senate, by subjecting it to such intense partisanship and actually allowing partisanship, to supersede the constitutional obligation, it's discouraging," Earnest told reporters. "It's a precedent that I think Republicans will regret setting."

Garland's nomination may have lapsed, but for his supporters, the wounds are fresh.

"Folks know that Republicans stole this seat and are extraordinarily disinclined to let them get away with it," said Drew Courtney, spokesman for the People for the American Way.

Courtney says that anyone on Trump's list of 21 potential Supreme Court nominees will provoke a big fight and that Republicans are mistaken if they believe that Democrats would accept someone off a "wish list" compiled by conservative groups such as the Federalist Society and the Heritage Foundation.

"Everyone understands that the stakes are enormous," he said.

It was back in 2013 when then-Senate Majority Leader Harry Reid, frustrated by Republican efforts to block lower court nominees, changed the rules and lowered the threshold to overcome a filibuster from 60 to 51 votes. Supreme Court nominees were exempt from the rules change and are still subject to the higher vote threshold. In his interview with MSNBC, Schumer signaled a hardline: that unless Trump nominated a "mainstream" candidate, Democrats might force Republicans to consider a rules change for Supreme Court nominees.

Schumer said the Republicans may not have the 60 votes "to put in an out of the mainstream nominee, and then they will have to make a choice: change the rules. It's going to be very hard for them to change the rules, because there are a handful of Republicans who believe in the institution of the Senate and don't change the rules."

Pressed on whether he'd work to hold the seat open, he replied, "Absolutely."

Republicans, meanwhile, praise Senate Majority Leader Mitch McConnell for following through with a promise to allow the "people" to choose Antonin Scalia's replacement after the election and they say every name on Trump's list of 21 is eminently qualified to take the bench.

Most of all, they are keenly aware of the stakes and are committed to ensuring that the new nominee might become Trump's most lasting legacy.

To be sure, while Trump's other nominees could serve for four years -- or even eight -- his Supreme Court nominee could spend decades on the bench. Justice Anthony Kennedy, for example, was placed on the bench six presidents ago.

"Donald Trump will pick from his excellent list of principled and highly qualified candidates who are committed to an interpretation of the Constitution as it was written," said Carrie Severino of the conservative Judicial Confirmation Network.

Trump ally Newt Gingrich rejects the notion that the Sessions hearings will impact the Supreme Court fight one way or another. "First of all, I think Sessions will be fine -- most of the attacks on him are just false," Gingrich said Tuesday

Gingrich added he thought that Trump would not stray from the list of 21 potential nominees he released during the campaign.

"I believe he has a moral obligation to name somebody who literally is on the list and was released in the campaign," he said.

New playing field

It was during the campaign that Scalia suddenly died and the White House put forward Garland, whom the President called a consensus candidate. Progressive groups supported the pick vigorously even though some had hoped for a different candidate who might have done more to rally the base. Throughout the spring they ran a campaign called #WENEEDNINE to push for hearings. By summer, Garland's name was no longer a headline.

But progressives hope that will change in the next few weeks.

"This is a moment in time for the broadest possible coalition of groups to go out to their members, affiliates and supporters to talk about the importance of the Supreme Court," said Nan Aron of the progressive Alliance for Justice.

"In the background lurks this very fixed idea that we are looking at a stolen seat, a seat that should have been filled by an Obama nominee who was forward looking not backward in his view of the Constitution," she said.

But building a coalition will be complicated and could take careful choreography. Unlike confirmation battles for a typical cabinet post, there are a variety of issues at stake and diverse perspectives.

They include the hot button issues -- abortion rights, voting laws, environmental regulations, union rules, campaign finance, gun control --that often closely divide the high court.

Christopher Kang, the National Director of the National Council of Asian Pacific Americans, has attended the meetings in preparation for the Sessions hearings and says it has been an important exercise for the groups to react without a package of material they might have come from the White House during the Obama administration. "This time it is up to us to build the record," he said.

Feinstein in charge

Sen. Patrick Leahy, a veteran of confirmation battles for years, is no longer the top Democrat on the Judiciary Committee, having moved over to Appropriations. Feinstein will take the helm, bringing with her some new staff.

Progressives note that Feinstein was quick to act when Sessions left off some key information -- his failed nomination as a federal judge -- when he submitted his first questionnaire. She fired off a letter directly to Sessions asking him for more information. In a statement, she did not ask for a delay in the Sessions hearings but asked Chairman Grassley to join a request for information she felt was missing.

"I am sure you would agree that staff must have sufficient time to do the due diligence on any nominee for this vital position," she wrote.

But some progressives are concerned that she might work differently from Leahy. They were surprised, for instance, in 2007 when she voted in favor of Judge Leslie Southwick a nominee of George W. Bush for the 5th Circuit.

But Feinstein voted against Bush's nominees to the Supreme Court, Chief Justice John Roberts and Justice Samuel Alito.

For now, before a nominee is announced, Elizabeth Wydra of the progressive Constitutional Accountability Center says that the "unprecedented" behavior of Senate Republicans last year "severely undermined bedrock norms of constitutional government."

"Now that Republicans are in control, they could take a first step in repairing some of the damage they inflicted by nominating a consensus candidate, in the mold of Garland," she said.