Roger Stone Roger Jason StoneThe agony of justice Our Constitution is under attack by Attorney General William Barr Justice IG investigating Stone sentencing: report MORE, a former adviser to President Trump, is now fundraising off a promise not to testify against him in special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE's investigation into Russian interference in the 2016 election.

Trump in recent days has lashed out at witnesses cooperating with the Justice Department's probe, such as his former attorney Michael Cohen, whom he's called a "rat."

Stone, who is being investigated by Mueller, wrote in conservative writer Ben Shapiro's "Call to Action" newsletter on Tuesday that he is refusing "to bear false witness against President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE" and that he "will not be railroaded by Mueller and his legal thugs."

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"I intend to fight any effort to frame me for perjury or act on false accusations against our President. But I urgently need your help," he continued, adding that "Mueller's illegal leaks have destroyed my business" and claiming that the cost of attorneys has "virtually bankrupted me."

He then asked supporters to make an "emergency contribution" of "$25, $50, $100, $500 or even $1,000" to his legal defense fund.

"Anything you send will be deeply appreciated," he added. "I intend to fight for my life but cannot do so without your help."

Stone did not immediately return a request for comment from The Hill.

Mueller is looking into whether Stone knew in advance that WikiLeaks planned to publish hacked emails from 2016 Democratic presidential nominee Hillary Clinton Hillary Diane Rodham ClintonDemocratic groups using Bloomberg money to launch M in Spanish language ads in Florida The Hill's Campaign Report: Presidential polls tighten weeks out from Election Day More than 50 Latino faith leaders endorse Biden MORE's campaign and the Democratic National Committee during the 2016 campaign.

Stone has denied having any advance knowledge of the email dumps but made several public comments in 2016 that seemed to forecast that the emails were coming.

Trump has praised Stone for saying that he will refuse to testify, writing on Twitter earlier this month that Stone was "essentially stating that he will not be forced by a rogue and out of control prosecutor to make up lies and stories."

"Nice to know that some people still have 'guts!'" Trump added.