When President Trump announced a ban on travel for citizens from several predominantly Muslim countries in January, a coalition of officials from various blue states quickly rallied to fight it.
"We just started talking to each other Friday afternoon," recalls New York Attorney General Eric Schneiderman. "By Sunday morning, we had 17 states signed on to say, 'This is unconstitutional. We're going into court to stop it.' And we went into courts all over the country and eventually got it struck down."
Throughout U.S. history, states have traditionally acted as counterweights to presidential authority, challenging the way laws are enforced. It was the states, for example, that waged some of the earliest and most effective legal battles against the Affordable Care Act.
And as the investigation into Russian interference in last year's election heats up, the states could once again play a powerful adversarial role, especially if the White House takes steps to thwart the probe.
If federal officials decide not to pursue charges — or are blocked from doing so — states conceivably could step in to fill the breach by filing charges of their own.
"There's no question that legally they can, if they have the ability to charge; in other words, if Trump or any of his associates have committed a crime in a particular state," says Jennifer Rodgers, executive director of the Center for the Advancement of Public Integrity at Columbia University Law School.
As Trump's home state, New York has a potentially important role to play, says Rodgers, who previously worked at the U.S. Attorney's Office.
"If you're talking about New York state, I think the likelihood is pretty good that if a federal crime was committed here that a state crime will also have been committed, because the campaign was based here. So probably lots of their meetings and phone calls and other actions like that will have occurred here," Rodgers says. "So then you're just looking at whether state law provides an avenue to charge whatever it is that the feds would be charging."
Importantly, while the president can pardon people accused of federal crimes, he has no such power at the state level.
"If there is a prosecution by a state, whether it's New York or any other state, the president of the United States and his pardon power does not prevail," says former New York Attorney General Robert Abrams.
New York also has the power to shut down corporations headquartered in the state, including the Trump Organization, if they are found guilty of crimes, although such a move would be a legally drastic one.
"The attorney general in New York is a very key player with very strong powers, particularly as it relates to corporate action and corporate entities, and can do an awful lot in terms of taking action to investigate, to prosecute, to penalize and even seeking to dissolve," Abrams says.
Schneiderman already has something of a history with Trump, having launched fraud charges against Trump University, a real estate seminar accused of fraud by some of its customers. Trump settled the case for $25 million shortly after his election last November, after long insisting he wouldn't do so.
"I do have more experience with the president than others, because I was in litigation with him for years over Trump University," Schneiderman says. "I didn't think this particular skill set would be so important, but it turned out, as of January, it was."
New York officials are already playing a part in the Russia investigation and have reportedly helped federal officials looking into money-laundering allegations against former Trump campaign manager Paul Manafort.
But Schneiderman says there has been a joint effort by a number of mostly blue states to block some of Trump's more controversial initiatives, in the areas of immigration, voting rights and the environment.
Some 15 states, including New York, Washington, North Carolina and Iowa, banded together earlier this month to file a lawsuit blocking the White House from terminating the Deferred Action for Childhood Arrivals program, which allowed some immigrants brought to the country illegally as children to stay in the United States.
The states are working together in a way that is much less ad hoc and more formalized than it used to be.
"There are regular conference calls. Staffs are talking to each other all the time. We're dividing up work. And there's much more of an attitude of, 'Look, we're all in this together,' " Schneiderman says.