Former Vice President Mike Pence plans to challenge a subpoena from the special counsel investigating former President Donald Trump’s role in the Jan. 6 riot, a source familiar with Pence’s plans told NBC News on Tuesday.

Pence’s legal team will fight the subpoena on the grounds that the former vice president was acting in his role as president of the Senate during the joint session of Congress on Jan. 6, arguing that he should be shielded from having to testify as part of the “speech or debate clause” that protects legislators from much outside scrutiny, the source said.

It is unclear how the clause prevents Pence from testifying about other conversations at or around the White House before the Capitol attack.

The former vice president might address the issue during a swing through Minnesota and Iowa on Wednesday, the source said.

Politico was first to report Pence’s plans.

NBC News reported last week that lawyers for Trump are expected to fight the special counsel’s subpoena of Pence on executive privilege grounds, according to a source familiar with the Trump legal team’s discussions.

Pence has been subpoenaed by special counsel Jack Smith, a source familiar with the matter told NBC News last week.

Attorney General Merrick Garland appointed Smith in November to lead the Justice Department’s inquiries into Trump’s role in the riot as well as the former president’s handling of classified documents after he left office. The subpoena of Pence is related to the Jan. 6 investigation, the source said.

Last December, NBC News reported that Smith subpoenaed local officials in key presidential swing states for all communications involving Trump, his campaign and a series of aides and allies who assisted in his efforts to overturn the 2020 election.

Smith’s move signaled that he was looking into a scheme involving Trump’s fake slate of electors who signed documents falsely asserting they were their states’ rightful electors and that Trump was the victor in those states.

The House committee formed in the last Congress to investigate the Capitol attack revealed evidence that the fake electors submitted false certifications of Trump victories to the National Archives in hopes of having Pence substitute them for the actual electoral votes that made Joe Biden president.

Trump attempted to claim executive privilege multiple times to block the House Jan. 6 committee from obtaining documents it had sought during its investigation.

The Jan. 6 committee dedicated an entire public hearing last year to detailing Pence’s role in presiding over the Senate on the day of the Capitol riot as well as the pressure campaign by Trump to get his then-vice president to interfere with the electoral count on Jan. 6.

In a Wall Street Journal opinion piece published in November, Pence said he told Trump that it would be illegal for him to interfere in the count during an Oval oOffice meeting with lawyer John Eastman, one of the architects of a memo that outlined a scenario in which Pence could refuse to certify the Electoral College count.

Pence ultimately carried out his constitutional duty to certify Biden’s electoral victory in the 2020 presidential election after the Capitol attack.