“Eric was very aggressive in challenging the Trump agenda in all sorts of ways, and it’s very difficult to predict what the next attorney general will pursue,” Mr. Koppell said. “I don’t know if the next attorney general will be equally aggressive.”
The end result of many of Mr. Schneiderman’s legal challenges still must be decided by the courts, but he certainly succeeded in at least temporarily slowing some key Trump administration policies.
Mr. Trump sparred with Mr. Schneiderman long before he became president, including about a civil investigation into Trump University that resulted in a $25 million settlement. Mr. Schneiderman had also opened an investigation into the Donald J. Trump Foundation in 2016 and then the Eric Trump Foundation in 2017. After Mr. Schneiderman’s resignation, allies of Mr. Trump cheered his demise.
“Gotcha,” tweeted Kellyanne Conway, counselor to the president, digging up an old tweet from Mr. Schneiderman in which he had reminded the president that “no one is above the law.”
Donald Trump Jr., the president’s oldest son, tweeted about Mr. Schneiderman no fewer than 10 times late Monday and early Tuesday.
Mr. Schneiderman had focused intensely on Mr. Trump since the day after the 2016 election, when he said he gathered staff members in Manhattan to discuss reorienting their whole operation. Since then, Mr. Schneiderman had engaged in dozens of lawsuits and administrative actions to challenge Mr. Trump from the earliest days of the administration.
So will Mr. Schneiderman’s abrupt exit leave a vacuum for Democrats, either politically or legally?
“The short answer is the work goes on,” said Sean Rankin, the executive director of the Democratic Attorneys General Association. “It does not slow down.”
Even though Mr. Schneiderman has often served as the public face of the sprawling legal battles against Mr. Trump, Mr. Rankin said that his organization had already devised a strategy to share responsibility for the various lawsuits against the president’s policies. Doug Chin, the attorney general of Hawaii, for instance, took the lead against Mr. Trump’s travel ban, Mr. Rankin said, and Xavier Becerra, California’s attorney general, stepped to the front in fighting back against the president’s attempts to repeal President Barack Obama’s signature health care law.
“States are protecting their people, values and economy, and that will continue as leaders throughout the country continue to emerge to resist and persist,” Mr. Becerra said in a statement.
“Nothing is going to change,” predicted James E. Tierney, a former attorney general of Maine who is now a lecturer at Harvard Law School. “No change — zero.”
Mr. Tierney added that while the legal actions taken by Mr. Schneiderman and his colleagues were filed in good faith, their ultimate success could reside in the eye of the beholder, and they might eventually be little more than speed bumps on the White House’s agenda.
Mr. Schneiderman’s federal suit in Brooklyn, for example, challenging Mr. Trump’s repeal of DACA, an Obama-era program that protects young undocumented immigrants, has already helped to keep the initiative in place, though it could eventually fail in the United States Supreme Court. And his litigation against the president’s first travel ban caused Mr. Trump to twice revise the policy, though it, too, might be unsuccessful in the end.
“Schneiderman and the other A.G.s are saying, ‘Look, we have a job to do. We can makes real arguments in real cases,’” Mr. Tierney sad. “And whether they ultimately win or lose, these are real cases.”
Still, the attorney general of New York, the home to Mr. Trump’s businesses, his family and his campaign, is uniquely well suited to playing an outsize role in legal matters involving the president.
In particular, Mr. Schneiderman had positioned his office as a fail-safe in the special counsel’s inquiry. In recent weeks, Mr. Schneiderman had sought to exempt New York’s double-jeopardy law from cases that involved presidential pardons.
But now he is the subject of a criminal investigation, begun Tuesday by the Manhattan district attorney, Cyrus R. Vance Jr. The future of that inquiry remained in doubt after Gov. Andrew M. Cuomo said he was considering appointing a special prosecutor to look into the charges.
For the time being, the office of attorney general will be taken over by Barbara Underwood, who has served as New York’s solicitor general for a decade, and who is respected in the legal community, though not seen as particularly political.
“She is not someone who has had experience rushing up to a political podium to inveigh against the so-called forces of evil,” said Seth W. Waxman, who worked with Ms. Underwood when he served as solicitor general under President Bill Clinton.
With Mr. Schneiderman out and Ms. Underwood in, some Democratic attorneys general from across the country discussed the new legal landscape on their regular weekly call on Tuesday.
“The A.G.s and I spoke about her today,” Mr. Rankin said. “She’s got a tremendous reputation. They’re looking forward to working with her.”
She could also be not long in the job: State legislators gathered in Albany on Tuesday, less than 12 hours after Mr. Schneiderman announced he would step down, to discuss whom they want to appoint to fill out his term.
A long list of Democrats have already begun jockeying behind the scenes to either win appointment or run for one of the plum jobs in New York politics. The primaries will be held in September.
Mr. Wittes, the Brookings Institution fellow, said playing the role of anti-Trump would most likely appeal to any Democrat who became attorney general. “Using that position as a launching pad for higher political ambition by opposing Trump is going to be an irresistible opportunity for some Democratic lawyer,” he said.
For Mr. Trump, the resignation represented a welcome end to an elected official he donated to in 2010 but turned against as Mr. Schneiderman began to investigate his business practices, calling him “incompetent,” a “hack” and a “shakedown artist.”
In 2013, he compared Mr. Schneiderman unfavorably to two other Democratic politicians felled by scandal: former Representative Anthony D. Weiner and former Gov. Eliot Spitzer. “Weiner is gone, Spitzer is gone — next will be lightweight A.G. Eric Schneiderman,” Mr. Trump wrote then. “Is he a crook? Wait and see, worse than Spitzer or Weiner.”
As a result of the insults, investigations and lawsuits, Mr. Schneiderman emerged as the most visible attorney general taking on Mr. Trump — much to the delight of Democrats.
“You are a superhero,” the late-night comedian Samantha Bee told him on her show last fall. During the segment, she featured Mr. Schneiderman on a series of faux comic book covers battling Mr. Trump’s agenda.
“With great power,” Mr. Schneiderman quoted the famous Spider-Man line back to her, “comes great responsibility.”