BREAKING: US Supreme Court Chief Justice Roberts takes oath to preside over the US Senate during the impeachment trial against President Trump. pic.twitter.com/LI676olHVC — NBC Politics (@NBCPolitics) January 16, 2020

Roberts, under the Constitution, is required to preside over a president's impeachment trial. Though senators say they expect his role to be largely ceremonial, he'll also have to weigh in on motions made by House managers, Trump's legal team and senators — though a majority of the body could overrule him.

Underscoring the pomp of the impeachment trial before Roberts was sworn in, the Senate did a "live quorum" where the Senate clerk called the name of every senator. As their name was called, senators stood from their desks and said they were "present."

Roberts is expected to also swear in each of the 100 senators.

The ceremonial housekeeping comes before the Senate will start the trial in earnest on Tuesday, when senators are slated to debate and pass a resolution establishing the rules of Trump's impeachment proceeding.

Opening arguments for Trump's trial are also expected to commence next week.