Kamala Harris could preside over Trump impeachment if Chief Justice John Roberts refuses

If the incoming vice-president also declines, the longest serving Democratic senator would likely fill the role

Kamala Harris could preside over Donald Trump's impeachment trial in the Senate if Chief Justice John Roberts declines to participate.

The US Constitution calls for the chief justice of the US Supreme Court to preside over presidential impeachment trials. However, the document is unclear regarding what happens if a president has left office by the time their trial begins.

According to Politico, Justice Roberts has said he "wants no further part" in the proceedings against the president. Mr Roberts presided over Mr Trump's first impeachment trial.

If he does not take the case, the president of the Senate would then determine who would rule in the case. Because Senate Majority Leader Mitch McConnell declined to call a special Senate session to begin the trial of Mr Trump, the proceedings will not begin until after Joe Biden has been inaugurated.

That means the president of the Senate will be Ms Harris.

She could choose to oversee the trial herself. If she does not, the job would likely fall to another Senator, likely to the longest serving Democrat in the Senate, Senator Patrick Leahy.

Justice Roberts has previously made clear his desire to keep the Supreme Court out of highly political battles, preferring instead to leave those decisions to the Congress.

During Mr Trump's first impeachment, Justice Roberts refused to cast a tie breaking vote in the Senate when Senate Democratic Leader Chuck Schumer asked him if he would do so.

"If the members of this body, elected by the people and accountable to them, divide equally on a motion — the normal rule is that the motion fails," Mr Roberts said. "I think it would be inappropriate for me, an unelected official from a different branch of government, to assert the power to change that result so that the motion would succeed."

Mr Trump was impeached by the House for inciting an insurrection at the US Capitol on 6 January.

Prior to his impeachment, House Speaker Nancy Pelosi gave vice president Mike Pence the option to remove Mr Trump from office by invoking the 25th Amendment and naming the president unfit to carry out his duties.

Mr Pence declined to do so, and Ms Pelosi moved on the impeachment.

Mr Trump is the first president in US history to be impeached twice.