One attorney general, Eric T. Schneiderman, is already investigating Donald 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.