EXCLUSIVE: Over 30 Senate Republicans are calling on Attorney General Merrick Garland to give the prosecutor investigating Hunter Biden special counsel authority as the investigation into the president's son heats up. 

In a letter led by Sen. John Cornyn, R-Texas, Judiciary Committee Ranking Member Chuck Grassley, R-Iowa, and Minority Leader Mitch McConnell, R-Ky., the senators warn of potential political interference in the high-profile probe being conducted by Delaware U.S. Attorney David Weiss, a prosecutor appointed by former President Donald Trump. The senators plan to send the letter later Monday, Cornyn's office told Fox News Digital. 

"As detailed by Senator Grassley, ‘highly credible’ whistleblower have come forward to detail a ‘widespread effort within the FBI to downplay or discredit negative information about’ Hunter Biden," the letter reads. "Instead of encouraging FBI and DOJ whistleblower to report crimes and promote government transparency, you took the inexplicable step of chilling lawful whistleblower activity."

It continues: "Indeed, on August 30, 2022… you issued a memorandum prohibiting DOJ and FBI employees from communicating with ‘Senators, Representatives, congressional committees, or congressional staff without advance coordination, consultation and approval by OLA.’"

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The letter comes after a source told Fox News in July the Hunter Biden investigation has reached a "critical stage," with officials weighing possible charges for tax violations, possible foreign lobbying violations and more.

John Cornyn speaks to reporters

Sen. John Cornyn (R-TX) speaks to reporters ahead of a weekly Republican luncheon on Capitol Hill on June 22, 2022 in Washington, DC.  (Photo by Brandon Bell/Getty Images)

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When the Biden administration took power, the administration asked Weiss to stay in his role even amid turnover at many other U.S. attorney positions. This was to avoid the appearance of political interference. 

The group of Senate Republicans led by Cornyn says that is not enough. 

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Attorney General Merrick Garland in Senate testimony earlier this year indicated that he is comfortable with how the investigation into Hunter Biden is proceeding without a special counsel in place. 

"Given the politicization of the DOJ under your watch and the importance of avoiding any appearance of impropriety, the undersigned request that you provide U.S. Attorney Weiss the full protections and authorities of a special counsel," the letter said. "This is one important action that you can take that will go a long way to restoring faith in our governmental institutions."

Garland in Senate testimony earlier this year defended the Justice Department's handling of the investigation, particularly in leaving Weiss, a Trump appointee, in charge of it. The attorney general, however, declined to specifically address the issue of whether to appoint a special counsel to look into the younger Biden and his business affairs. 

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The federal investigation into Hunter Biden and his tax affairs has reached a "critical stage," a source told Fox News this summer, as officials are looking into whether to charge President Biden’s son with various tax violations, possible foreign lobbying violations and more. ((Randy Holmes via Getty Images))

"The investigation is being run and supervised by the United States attorney in Delaware who is an appointee of the previous administration, and continues on as the United States attorney," Garland said under questioning from Sen. Mike Braun, R-Ind. 

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"The question of whether to have a special counsel is one that's internal decision-making in the department so I don't want to make any judgments one way or another," he added. "But I'm quite comfortable with the United States attorney for that district continuing in the role he's playing."

Fox News' Brooke Singman, David Spunt and Bill Mears contributed to this report.