While running to be New York’s attorney general, Letitia James did not mince words about Donald Trump. She called him an “illegitimate president” who should be removed from office, and vowed to use every legal avenue to investigate Trump and his business dealings.

Since taking office at the beginning of the year, she has toned down the rhetoric but she has let subpoenas do the talking instead – pursuing a barrage of investigations and emerging as a major thorn in the side of Trump and his political allies and notching up successes where even Congress has been blocked.

Trump himself has taken notice. After the attorney general launched an investigation into the National Rifle Association, Trump said in a tweet that the gun group is “under siege” by James and New York governor Andrew Cuomo, and accused them of “illegally using the state’s legal apparatus to take down and destroy this very important organization, & others”.

Earlier, he complained that James “openly campaigned on a GET TRUMP agenda” and called her investigations “part of the Witch Hunt Hoax”.

The attorney general has issued subpoenas as part of an investigation into the non-profit status of the NRA, where an internal power struggle has exposed allegations of financial mismanagement.

That came after her office launched an investigation into Trump’s own finances, sending subpoenas to Deutsche Bank, which has made loans to the president’s businesses. The investigation was prompted by former Trump lawyer Michael Cohen’s testimony that Trump falsely inflated his assets while seeking loans.

“We follow the facts and the evidence wherever it leads, and no one is above the law,” James said last week at her lower Manhattan office. “Including powerful organizations such as the NRA. Including the most powerful individual in this country, the president of these United States. It’s really about the rule of law.”

Unlike similar subpoenas from Congress, which Trump and his children sued the bank to block, there has been no effort so far to quash the attorney general’s subpoenas, and the bank has begun turning over documents, James said. She hopes to review Trump’s tax returns as well.

James is also pursuing legislation to change the state’s double jeopardy laws, so that any Trump associates pardoned by the president for federal crimes could be charged on the state level.

Her lawyers argued before the supreme court in a lawsuit seeking to stop the administration from adding a question on citizenship to the US census, and investigators are investigating complaints of labor violations at the Trump National Golf Club Westchester in the New York suburbs.

Democratic attorneys general across the nation have fought Trump with scores of lawsuits, but the powers of James’s office and her perch in the president’s home state positions her as a unique threat.

“Trump has decades of complex and shady business deals that make for a target-rich environment,” said Eric Soufer, who was a senior counsel in the attorney general’s office before James took over.

The state’s financial fraud laws give the attorney general “incredible power” to demand documents and records, said Paul Nolette, a political science professor at Marquette University.

“New York is in a unique position to lead a lot of these investigations,” he said. “Number one, they have the resources to take Trump on, and number two, they have the location where Trump is potentially most legally vulnerable.”

New York has joined 51 multi-state lawsuits against the Trump administration, including 26 where it has led the group – the most of any state, according to Nolette’s count.

When James took office in January, she became the first black woman to serve as attorney general – and the first to hold any statewide office in New York.

A Brooklyn native, she was New York City’s public advocate, a job that has virtually no formal power, but has often been a springboard to higher office. Mayor Bill de Blasio was also public advocate before he won his current job.

James had been eyeing the mayor’s office too, until the resignation of Eric Schneiderman, accused of violent behavior against multiple women, created a sudden vacancy for attorney general.

She jumped into the race as the early front runner, although some of her one-time allies on the left, put off by her embrace of Cuomo, flocked to law professor Zephyr Teachout instead. In the end, James prevailed in a four-way primary, and easily defeated her Republican opponent.

As some progressives look to James as their next best hope to hold Trump accountable after the conclusion of the Mueller investigation, conservative critics say her approach is excessively partisan for a law enforcement office and she should focus on troubles closer to home.

“She is trying to make a name for herself as a partisan,” said Joe Borelli, a Republican city councilman and one of the few prominent Trump supporters in New York politics. “If she cared about public corruption, she would be investigating the governor of her own state, whose top aides and allies are now in prison and who disbanded his own investigative commission when it began to look at him.”

Trump lawyers have taken the feud beyond Twitter, arguing in court papers that a suit against Trump’s charitable foundation should be rejected because it is “the product of the attorney general’s animus and bias against President Donald J Trump and it was filed for improper, biased and political reasons”.

The foundation agreed to dissolve as part of a settlement with the AG’s office, which alleged its funds were misused for political and personal purposes, but James has continued to pursue it for damages and seek an order banning Trump and his children from running New York charities.

Some staffers in the AG’s office were “taken aback” by James’s comments during the campaign, Soufer said, but he added she has conducted the investigations responsibly since taking office.

“All of the investigations and work around the Trump administration have been taken on in a very credible way. I don’t think anyone can credibly accuse the office of overreaching in any of those areas,” he said.