One attorney general, Eric T. Schneiderman, is already investigatingDonald J. Trump over possible violations of New York State law at his charity foundation.
Another, Maura Healey of Massachusetts, has joined Mr. Schneiderman in an investigation into whether Exxon Mobil — whose chief executive, Rex W. Tillerson, is Mr. Trump’s choice for secretary of state — lied to investors and the public about the threat of climate change.
Ms. Healey also has a new fund-raising pitch: “I won’t hesitate to take Donald Trump to court if he carries out his unconstitutional campaign promises,” she recently wrote to supporters.
A third, Representative Xavier Becerra, who was chosen this month to become California’s attorney general, has dared the Trump administration to “come at us” over issues including immigration, climate change and health care.
As Democrats steel themselves for the day next month when the White House door will slam on their backs, some of the country’s more liberal state attorneys general have vowed to use their power to check and balance Mr. Trump’s Washington.
If the Trump administration withdraws from environmental, antitrust or financial regulations, the attorneys general say they will plug regulatory holes that may gape wide open, deploying state laws like New York’s Martin Act, which allows the state attorney general to pursue wide-ranging investigations on Wall Street.
They have pledged to defend undocumented immigrants, and to combat hate crimes that many believe were unleashed by Mr. Trump’s polarizing campaign.
And if Mr. Trump’s policies veer toward the unconstitutional, several of the 10 current and incoming Democratic attorneys general interviewed recently said they would apply a remedy favored by Mr. Trump himself: a lawsuit.
The strategy could be as simple as mirroring the blueprint laid out by their Republican colleagues, who made something of a legal specialty of tormenting President Obama. Conservative attorneys general in states including Texas, Virginia and Florida have sued the Obama administration dozens of times, systematically battering Mr. Obama’s signature health care, environmental and immigration policies in the courts.
One of them, Scott Pruitt, the attorney general of Oklahoma, who used his office to bayonet Mr. Obama’s clean-energy regulations, was just chosen by Mr. Trump to become the new head of the Environmental Protection Agency.
Mr. Schneiderman — who established himself early as a nuisance to Mr. Trump when he sued him over Trump University, negotiating a $25 million settlement — pounced on the Pruitt selection, calling him “an agent of the oil and gas industry” and promising to push an E.P.A. under Mr. Pruitt to uphold environmental laws. Ms. Healey has also expressed concern about the nominations of Mr. Pruitt and Mr. Tillerson.
The jockeying to begin hostilities with the Trump administration is a measure of how the country’s widening political divide has transformed the offices of state attorneys general into legal laboratories and sharpened them into political scalpels.
They were once primarily local law enforcement figures who rarely pursued issues beyond state borders. But with the growth of their clout and ambition over the last three decades, they have become magnets for lobbyists, campaign donors and other corporate representatives looking to intervene in regulatory policy and tip investigations, a New York Times investigation found in 2014.
Under President Bill Clinton, attorneys general pioneered the major multistate lawsuit that has served as a model for interstate collaboration since, with nearly all the states joining together to win a groundbreaking settlement with the tobacco industry. Liberal states later collaborated to force the E.P.A. under President George W. Bush to regulate greenhouse gas emissions, winning a Supreme Court decision that made it easier for the states to sue the federal government.
It was under Mr. Obama that states came into their own as political activists. One group of Republican attorneys general began holding weekly conference calls to strategize ways to weaken the Affordable Care Act months before it became law in March 2010, filing their lawsuit minutes after Mr. Obama signed the bill.
For the moment, the precise shape of Trump-branded targets is hard to make out. At the annual meeting of the National Association of Attorneys General in Fort Lauderdale, Fla., two weeks ago, bipartisan bewilderment about the president-elect’s true intentions abounded. (Republican state attorneys general will slightly outnumber Democrats in 2017.)
“People are coming up to me and saying, ‘What’s going to happen?’” said James E. Tierney, a former attorney general of Maine, who ran a program studying attorneys general at Columbia Law School. Mr. Tierney, a Democrat, now lectures at Harvard Law School. “There’s a lot of eye-rolling down here, in both parties, like, ‘Oh my God.’”
Even as attorneys general from blue strongholds like California, Massachusetts and New York hasten to brand themselves as leaders of the opposition, many of their Democratic colleagues are striking a less antagonistic note as they wait to see how Mr. Trump will govern.
One incoming attorney general, T.J. Donovan of Vermont, said he was ready to collaborate with the federal government to tackle the heroin epidemic in his state, among other issues — and to dissent when necessary. “Let’s be patient and wait and see what happens,” he said. “But at the same time, let’s be prepared.”
Mr. Schneiderman, it seems, is not straining to give Mr. Trump the benefit of the doubt. Since Election Day, he said in an interview, he has spoken to several attorneys general about teaming up, as Democratic attorneys general have already done to defend Mr. Obama’s clean-power plan against Republican legal challenges.
“Life just got a lot more exciting for those of us at the state level who are now the first line of defense,” said Mr. Schneiderman, adapting a favorite catchphrase of Republican attorneys general.
Their litigiousness turned attorneys general like Greg Abbott, now the governor of Texas, into right-wing luminaries. He often rallied crowds by saying, “I go to the office in the morning, I sue Barack Obama, and then I go home.”
Next year, there is likely to be no shortage of Democrats who can say the same about Mr. Trump.
The states’ rights arguments that Republicans have made gospel for nearly eight years — that states must serve as a check against federal overreach — are likely to become convenient for Democrats. So are the legal tactics that Republican attorneys general used to stifle Obama administration programs, including filing lawsuits in front of friendly local judges to win nationwide injunctions against policies they hoped to stop, said Amanda Frost, a professor at American University’s Washington College of Law.
With Mr. Trump’s ascension, attorneys general of both parties may shuck any remaining veneer of nonpartisanship, even as they continue to wade across party boundaries on investigations involving consumer protection or pharmaceutical pricing.
According to Paul Nolette, a political-science professor at Marquette University, who studies attorneys general, Republican attorneys general filed partisan legal briefs in only five Supreme Court cases during the Clinton administration, a figure that rose to 97 in the first seven years of the Obama administration.
“Things are being driven more by partisan politics,” Mr. Nolette said. “On virtually every hot-button issue you can imagine, A.G.s are signaling where they stand.”
As Mr. Abbott and Mr. Pruitt found, there are certain advantages to occupying a high-profile law enforcement office with an anti-Washington megaphone.
Josh Shapiro, the incoming Democratic attorney general of Pennsylvania, said he had turned down a run for the Senate this year in favor of the attorney general race. “I believe it to be the most impactful job in government today,” he said.
Other Democrats said they were watching how Mr. Trump would treat the Consumer Financial Protection Board and the Federal Trade Commission.
Under Mr. Obama, attorneys general have grown used to working closely with both agencies on consumer and antitrust issues — “It’s been the golden years,” said Tom Miller, the longtime Democratic attorney general of Iowa — and several said they feared federal regulatory might would shrivel under the new administration, leaving states to try to hold the line with far fewer resources.
“I don’t want to pick fights before there are fights,” said Brian Frosh, the Democratic attorney general of Maryland. “But based on the campaign, there’s cause for concern.”