Former Vice President Mike Pence is preparing to resist a Justice Department grand jury subpoena for him to testify on former President Donald Trump’s efforts to overturn the 2020 election results, Fox News has learned.

A source familiar with the former vice president's thinking told Fox News on Tuesday morning that Pence plans to fight the subpoena issued last week by Special Counsel Jack Smith regarding the Jan. 6 investigation into Trump, on the ground that he was president of the Senate and should be shielded from the order. Currently, it appears Pence is not planning to assert executive privilege.

Politico first reported on Pence’s decision to challenge the DOJ subpoena earlier Tuesday, citing two people familiar with the former vice president’s thought process. 

MIKE PENCE'S FORMER ADVISER SAYS DOJ SUBPOENA ACTS AS ‘SECURITY BLANKET’ FOR EX-VP'S POSSIBLE 2024 RUN 

The former vice president’s allies told Politico he's protected by a constitutional provision called the "speech and debate clause," which generally shields members of Congress from having to fear that anything they say during legislative activities will implicate them in a lawsuit. According to Politico, Pence’s allies said federal prosecutors are legally blocked from compelling testimony from the former vice president related to key elements of Smith’s probe under the Constitution’s separation of powers. 

Mike Pence fists

Former Vice President Mike Pence is reportedly to fight Justice Department Special Counsel Jack Smith’s subpoena, citing the speech and debate clause.  (John Lamparski/Getty Images)

"He thinks that the ‘speech or debate’ clause is a core protection for Article I, for the legislature," one of the two sources familiar with Pence’s legal strategy told Politico on condition of anonymity. "He feels it really goes to the heart of some separation of powers issues. He feels duty-bound to maintain that protection, even if it means litigating it."

Garland taps special counsel for Trump probes

U.S. Attorney General Merrick Garland announced he will appoint a special counsel to oversee the Justice Department’s investigations into former President Donald Trump on Nov. 18, 2022, in Washington, DC.  (Anna Moneymaker/Getty Images)

Pence is the latest official in Trump's administration to be subpoenaed as part of the investigation into efforts to overturn the results of the 2020 presidential election, but the push for Pence's testimony is unique because he's the highest-ranking official known to have been summoned.

Trump, for his part, has not said whether he plans to assert executive privilege, a doctrine meant to protect the confidentiality of the Oval Office decision-making process, to prevent Pence's testimony. 

Trump and Pence

Donald Trump's lawyers have not said if they will intervene to help fight the DOJ subpoena seeking testimony from former Vice President Mike Pence.  (Win McNamee/Getty Images)

Former Trump administration national security adviser Robert O'Brien has also been subpoenaed by the special counsel as part of the Jan. 6 investigation and a separate probe into the presence of classified documents at Trump's residence at Mar-a-Lago in Florida, according to several media reports last week.

This past Friday, the FBI discovered an additional document with classified markings at Pence's Indiana home following the discovery by his lawyers last month of sensitive government documents there. The search was described as consensual after negotiations between Pence's representatives and the DOJ. 

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U.S. Attorney General Merrick Garland tapped Smith as special counsel to oversee the investigations into Trump last November shortly after the former president announced that he will seek the Republican ticket again in 2024. Last month, Garland chose a separate special counsel, Robert Hur, to investigate the classified documents found at the home and former office of President Biden. 

The Associated Press contributed to this report.