Hunter Biden's attorney Abbe Lowell, who out of the gates has questioned the legitimacy of Republicans oversight investigation and has threatened lawsuits against private individuals, including former President Trump and Rudy Giuliani, has aided Democrats more than just publicly.

The high-powered partner in the Washington, D.C., office of Winston & Strawn LLP is also a prolific Democratic donor and has showered liberal campaigns and political action committees with more than $100,000 in contributions over the years.

Biden tapped Lowell, one of the nation's premier white-collar defense attorneys, whose past clients have included New Jersey Sen. Bob Menendez, former North Carolina Sen. John Edwards, and Jared Kushner, to help him in his legal battle relating to his infamous abandoned laptop. 

WHO IS ABBE LOWELL? HUNTER BIDEN'S HIGH-PROFILE ATTORNEY IN THE LEGAL BATTLE OVER HIS INFAMOUS LAPTOP

Abe Lowell and Sen. Robert Menendez speak to reporters

High-profile attorney Abbe Lowell represented Sen. Robert Menendez, D-N.J. in the 2015 corruption trial. (Kena Betancur/Getty Image)

While Lowell has aided several Democrats publicly, he has also done so from his pocketbook. Lowell has given extensively to Democratic presidential candidates, including President Biden, former President Obama, and failed contenders Hillary Clinton and John Kerry, according to Federal Election Commission records.

His cash has also benefited numerous Democratic lawmakers, including New Jersey senators Cory Booker and Bob Menendez and Maryland representatives Steny Hoyer and Jamie Raskin.

Lowell came out swinging in his defense of Hunter Biden, telling the Republican-led House Oversight Committee on Thursday that their investigation "lacks a legitimate legislative purpose and oversight basis" after the committee's chairman, Kentucky Republican James Comer, requested records and communications from the younger Biden, his longtime business partner Eric Schwerin, and President Biden's brother, James Biden.

"As your Letter is a sweeping attempt to collect an expansive array of documents and communications from President Biden and his family, I write to explain that the Committee on Oversight and Accountability lacks a legitimate legislative purpose and oversight basis for requesting such records from Mr. Biden, who is a private citizen," Lowell responded.

HOUSE REPUBLICANS PRESS HUNTER BIDEN, JAMES BIDEN, ERIC SCHWERIN FOR BUSINESS DEAL DOCUMENTS 

Abbe Lowell

Abbe Lowell has provided more than $100,000 to Democratic donations over the years. (Scott J. Ferrell/Congressional Quarterly/Getty Images)

"Rather than engage in back-and-forth letter writing campaigns or any formal proceedings, I would offer to sit with you and your staff, including the ranking member and his staff, to see whether Mr. Biden has information that may inform some legitimate legislative purpose and be helpful to the Committee. I hope that you will engage in this effort," Lowell said.

Lowell has further urged prosecutors to launch state and federal investigations into Delaware computer repair shop owner John Paul Mac Isaac, whom he accused of "unlawfully" accessing the younger Biden's data on his laptop after it was left at his repair shop in 2019. 

The letter also lists Rudy Giuliani, Steve Bannon, and other notable Biden critics for their roles in allegedly disseminating the information to the public.

And on Wednesday, Lowell threatened potential lawsuits by sending a "litigation hold" letter to 14 individuals said to have allegedly generated "coverage critical of the 53-year-old son of President Joe Biden, according to a person familiar with the development," Politico reported

FORMER TWITTER EXECS WHO CENSORED HUNTER BIDEN LAPTOP STORY FACE QUESTIONING FROM HOUSE GOP

Hunter Biden, Joe Biden

Hunter Biden is facing a GOP-led House Oversight Committee investigation in relation to his infamous abandoned laptop. ((AP Photo/Patrick Semansky))

The individuals who received the letter include former President Trump, Rudy Giuliani, his lawyer Robert Costello and an attorney representing John Paul Mac Isaac.

"Abbe is not cheap, and you don't bring in Abbe unless you want to go to war or prevent one," a source who's worked with Lowell recently told Fox News Digital. "They're going on the offensive. He hasn't been charged with anything, but they're trying to prevent that, because that would be bad for [President] Biden and Hunter."

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"Abbe is one of the best attorneys on Planet Earth, so hiring him from a strategic standpoint was a brilliant move," they said. "You couldn't find someone who can navigate Congress and the DOJ space better than him."

Lowell did not respond to a Fox News Digital request for comment on his political donations. 

Fox News' Chris Pandolfo, Patrick Ward, Joseph A. Wulfsohn, and David Rutz contributed reporting.