Asked on Sunday on CBS's Face the Nation with Margaret Brennan if he would be a witness against Trump if the case goes to trial, Pence said he had "no plans" to testify, but did not rule it out.

"But people can be confident we'll obey the law, we'll respond to the call of law, if it comes, and we'll just tell the truth," said Pence, who is running against Trump in the 2024 Republican presidential nominating contest.

Pence became a central figure in Trump's latest criminal charges on Aug. 1 when a four-count, 45-page Justice Department indictment charged the former president with illegally trying to cling on to power after he lost the 2020 election to Democrat Joe Biden.

Trump pleaded not guilty to all the charges on Aug. 3.

In the run up to the certification of Biden's win in Congress on Jan. 6, 2021, Pence oversaw the proceedings in his ceremonial role as president of the U.S. Senate. He came under huge pressure from Trump to overturn the vote and refused. Some of the Trump supporters who rioted at the U.S. Capitol chanted "Hang Mike Pence!"

That incessant pressure, and the "contemporaneous notes" Pence took in the run up the Jan. 6, 2021 riot, are frequently cited in last week's indictment.

At one point, the indictment refers to a Jan. 1, 2021 phone call during which Trump berated Pence for not participating in the scheme to overturn the election result.

"You're too honest," Trump told Pence.

Pence's potential testimony and his notes could become key parts of the prosecution case against Trump at trial.

For the first time in the primary campaign, Trump attacked Pence on Saturday. He denied he called him "too honest", described him as "delusional", and called him "'Liddle' Mike Pence".

Pence's presidential campaign has failed to take off so far, and he even risks not making the first Republican presidential debate later this month because of a lack of donors.

He is stuck in low single digits while Trump has become the runaway frontrunner, his poll numbers boosted each time he gets indicted.

Trump has been indicted three times this year. He has pleaded not guilty in two other criminal cases. He faces federal charges in Miami for allegedly retaining classified documents after leaving office and obstructing justice, and New York state charges in Manhattan for allegedly falsifying business records to hide hush money payments to a porn star.

Trump could face a fourth indictment this week in Georgia, related to his efforts to overturn his defeat in the battleground state after the 2020 election.

Pence took on his former boss more forcefully last week, accusing him of surrounding himself with "crackpot" lawyers after his 2020 defeat.

Pence's campaign has sought to profit from Trump's indictment. Last week they began selling hats and T-shirts with a "Too Honest" logo.