Groups representing Saudi Arabia, Kuwait, Bahrain and the Philippines have hosted events at Trump's Washington hotel since he took office.
The President has long refused to publish his tax returns, so the new documents could be a rare insight into the workings of his real estate empire. While it's unlikely he will have to reveal any personal tax returns, lawyers have requested state and federal tax returns for the Trump Organisation and Trump's other business entities including The Donald J. Trump Revocable Trust.
The lawyers are aiming to prove that hotel revenues are going to the President through his affiliated entities, including his trust.
When Trump won the election, he placed his sons Eric and Donald Jr in charge of his businesses — but a trust agreement allows him to draw money from it upon his request.
Eighteen private entities including restaurants, venues and hotels that compete with the Trump hotel are also being subpoenaed to "illuminate the unfair nature of that competition," said Maryland Attorney-General Brian Frosh.
"He's received numerous payments from foreign governments and state governments and they've been funnelled, at least in part, through the Trump (hotel) in DC."
The Trump Organisation said in an emailed statement to the Associated Press that the company had "voluntarily donated" the profits to the Treasury and planned "to make a similar contribution in 2019".
But there is little transparency around how the US$210,000 ($305,249) figure for foreign government patronage was calculated.
Trump had tried to stall the lawsuit, with lawyers from his Department of Justice saying it would "be a distraction to the President's performance of his constitutional duties".
But the document requests went ahead on Tuesday.
Trump is accused of violating two snappily named "emoluments" clauses in the Constitution — meaning wage, fee or profit from employment or office.
One of the clauses covers business with foreign governments, and the other concerns federal officials profiting from financial relationships with individual states.
The case may sound technical, but it goes to the heart of one of the greatest ongoing accusations against the billionaire President, who presents himself as a man of the people.
That is the allegation his business and political aims overlap to an unacceptable, even illegal, degree.
In recent days, his former lawyer Michael Cohen caused embarrassment for Trump by admitting he had lied about his boss's plans to build a Trump Tower in Russia in 2016. At the time, Trump was the presumptive Republican presidential nominee, vowing to improve relations with Vladimir Putin, and refusing to condemn Russia for election meddling.
Others have asked whether his refusal to condemn Saudi Arabia for journalist Jamal Khashoggi's murder comes back to his own business dealings with the country — not just the vast sums they are spending on buying arms from the US — which the Democrats also plan to investigate.
There are also questions over how much money Trump's associates and family members have made from his tax cuts.
The case may not get as far as the President's detractors hope. It could be blocked in a higher court, or Trump's lawyers could stop the documents from being made public.
Even if a violation is proven, Trump may simply plead ignorance and change course.
What it will do is reveal more information about his secretive business dealings, providing more opportunity for his Democratic rivals to hold him to account and demand his personal tax returns.
With a new majority in the House of Representatives following the midterms, the Democrats are planning a series of investigations into the President, leaving him with little room for manoeuvre.
And his rival for the 2020 presidential election will be using everything they can find.
