In a 41-page opinion on Monday, Indian-American Judge Amit Mehta of the DC District Court dealt a significant blow to the White House as he rejected Trump's attempt to block the committee's subpoena, asserting that Congress is well within its authority to investigate the President, reports CNN.
Congress specifically can probe the President for conflicts of interest and ethical questions, Mehta wrote, reaching into history citing everything from the presidency of James Buchanan, to the Teapot Dome, Watergate and Whitewater scandals to back up his ruling.
"History has shown that congressionally-exposed criminal conduct by the president or a high-ranking Executive Branch official can lead to legislation," Mehta wrote.
"It is simply not fathomable that a Constitution that grants Congress the power to remove a president for reasons including criminal behaviour would deny Congress the power to investigate him for unlawful conduct past or present even without formally opening an impeachment inquiry."
Trump's legal team plans to appeal, an informed source told CNN later Monday.
Mazars won't have to turn over the subpoena for another seven days, according to the Judge.
The House Oversight Committee subpoena of Mazars seeks a vast amount of communications and financial records related to the firm's work for Trump himself, as well as his businesses and foundation from 2011 through 2018.
Mazars became a target in the House investigation after Trump's former personal attorney Michael Cohen accused the President of fudging his wealth in an unsuccessful attempt to buy the Buffalo Bills football team and reduce his real estate tax burden.
Trump immediately called the ruling "crazy".
"We disagree with that ruling, it's crazy," Trump told reporters on Monday evening before departing the White House for a rally in Pennsylvania, noting that Mehta was appointed by former President Barack Obama.
"This never happened to any other president," he said, adding that Democrats were trying to "get a redo" on Special Counsel Robert Mueller's probe into Russian meddling in the 2016 election.