Washington DC - Former US Vice president Mike Pence on Wednesday said he will testify in the January 6 investigation and agreed not to appeal a federal judge's order that he must talk under oath to the grand jury.

In a major breakthrough for special counsel Jack Smith, Pence said he would obey the ruling that rejected his effort to avoid testifying about former President Donald Trump's scheme to overturn the 2020 election.

It's not known when Pence might appear before the grand jury, and a separate long-shot appeal by Trump could still be pending.

"Pence will not appeal the judge's ruling and will comply with the subpoena as required by law," said Devin O'Malley, a spokesman for Pence.

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The 63-year-old will now be forced to talk under oath for the first time in coming weeks about Trump's unprecedented effort to stay in power after losing to President Joe Biden.

Even though Pence has discussed some of his interactions with Trump in his memoir and in interviews, he refused to appear before the congressional committee that investigated January 6 and initially dismissed Smith's subpoena as unconstitutional.

Although Trump's own lawyers could appeal the decision, chances of them doing so successfully are slim.

Pence is among a handful of the most important witnesses, in part because he had numerous closed-door meetings alone with Trump, who sought to persuade him to join the "Stop the Steal" campaign to overturn the election and avoid handing power peacefully to Biden.

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It's not fully known what tactics Trump may have used to cajole Pence into dropping his opposition to the election plot, which culminated with the January 6 attack on the US Capitol.

When Pence resisted and eventually rejected his demands, Trump turned on his longtime loyal lieutenant, deriding him as a traitor to the MAGA cause.

Trump ordered thousands of his supporters to march on the Capitol and "fight like hell" to keep him in power on January 6.

The violent extremists chanted "hang Mike Pence" and erected a mock gallows to carry out a potential assassination after Trump egged them on by calling Pence a coward in a tweet.

Along with the January 6 probe, Jack Smith is also investigating Trump's mishandling of classified documents that he took with him when he left the White House.

A Georgia prosecutor is looking into Trump's effort to overturn his loss in the Peach State.

None of the cases is related to Trump's indictment in the Manhattan hush money case that saw him on Tuesday become the first former president ever to face criminal charges.