Of the more than 20 congressional investigations into the president and his finances - which have led to subpoenas for records from places like Capital One and Deutsche Bank - a court battle over records from a little-known accounting firm could mark the first real test in the unprecedented standoff between President Donald Trump and the House Democrats investigating him.

A federal judge heard arguments on Tuesday and said financial records from Mazars - Trump's longtime accountant - would be part of a “proper subject of investigation” by Congress, appearing to side with Democratic lawmakers, who subpoenaed Mazars for the president's personal and business finances.

But a lawyer for Trump argued that the Democrats’ subpoena fell outside of Congress’ legislative function.

Reuters Legal Correspondent Jan Wolfe explains.

"Legal experts generally say that Congress has the upper hand here and is going to win.

That's because the Supreme Court has said that Congress' power to investigate is broad." The House Oversight Committee, which issued the subpoena, said that it needs the documents to properly examine whether Trump has conflicts of interest or broke the law by not disentangling himself from his business holdings the way past presidents have done.

Trump's lawyer on Tuesday said the House was claiming “unbridled” power to investigate Trump, saying: “They have made clear that this is not about legislation.

They want to know if there has been any wrongdoing.

That is not the purpose of Congress.” The judge on Tuesday said that he would issue a ruling on the subpoena soon.

But whatever the outcome, the ruling will almost certainly be appealed to a higher court.