Washington DC - Former Vice President Mike Pence has been ordered by a federal judge to testify about conversations with former President Donald Trump ahead of the January 6 attack on the US Capitol.

In a major victory for prosecutors, Pence has been told that he must answer questions about the attempted insurrection before a grand jury investigating Trump's effort to overturn his loss in the 2020 election.

District Court Judge James Boasberg did rule that Pence may be able to avoid answering some questions about his role as ceremonial president of the Senate during the certification of the presidential election results on January 6.

It wasn't immediately clear if Pence plans to appeal the decision or when he might have to testify.

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The judge also rejected a separate filing by Trump himself to block Pence's testimony on the grounds of executive privilege.

The blockbuster ruling issued Monday was reported by NBC News on Tuesday.

"There is no factual or legal basis ... to any case against President Trump," a spokesperson for Trump said in a statement.

The grand jury meets in secret, so any revelations might not immediately be made public unless Pence chooses to also speak openly.

Pence, who resisted Trump's scheme to stay in power after losing the election, has never spoken publicly about many aspects of the plan and the violent attack on the Capitol by thousands of extremist Trump supporters.

Pence refused to testify before the congressional select committee on January 6, citing his role as vice president, although he tacitly allowed his top aides to appear.

He invoked his other role as ceremonial president of the Senate in the failed motion to avoid having to obey a subpoena for testimony from special counsel Jack Smith.

Boasberg agreed that the Constitution's "speech and debate" clause can be used by Pence to avoid answering some questions specifically about his role as the president of the Senate in presiding over the certification of the election results.

Trump confronted Pence several times in the days leading up to January 6 in an unsuccessful effort to get him to join his so-called Stop the Steal campaign.

Some of those interactions were in private, meaning no one besides Pence knows what Trump said or what Pence said.

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After a final split on the morning of January 6, Trump issued a fiery call to supporters at a rally to "fight like hell" to keep him in power.

The rioters marched to the Capitol and overwhelmed outnumbered police. They rampaged through the building hunting down perceived enemies, including Pence.

They erected a mock gallows outside and chanted "Hang Mike Pence!"

As the rampage unfolded, Trump tweeted another attack on Pence, calling him a coward. Cassidy Hutchinson, a former White House aide, told the January 6 committee hearings the then-president told aides Pence deserved to be punished by the mob for being disloyal.

Forcing Pence to testify represents a huge victory for Smith, who has enjoyed a string of courtroom successes recently.

Another judge ordered ex-White House chief of staff Mark Meadows and several other top aides to testify, rejecting their claims of executive privilege, which sometimes can be used to shield presidential decision-making from scrutiny.