Former President Trump faced a deadline to submit a pretrial brief on Monday, a day before the Senate is due to begin his second impeachment trial on a charge of inciting insurrection arising from the deadly Jan. 6 rampage at the U.S. Capitol.

The nine Democratic House of Representatives lawmakers who will serve as prosecutors hope to persuade members of the 100-seat Senate to convict Trump and ultimately bar him from holding public office again. Trump ended his four-year term in office on Jan. 20, having lost the Nov. 3 election to Democrat Joe Biden.

Monday’s deadline for the filing by Trump’s legal team comes as the defense prepares to emphasize its argument – laid out in an earlier filing – that the Senate lacks the constitutional authority to conduct the trial now that he has left office and is a private citizen. Challenging the case against Trump on such grounds would enable his fellow Republicans in the Senate to vote against conviction without directly defending his inflammatory speech to supporters shortly before the riot.

The Democratic-led House impeached Trump on Jan. 13 on a single charge of inciting insurrection, focused on that speech. He is the first U.S. president to be impeached twice and the first to face trial after leaving office.

During that speech, Trump repeated false claims that the election was fraudulent and exhorted supporters to march on the Capitol, telling them to “stop the steal,” “show strength” and “fight like hell.” The rampage interrupted the formal congressional certification of Biden’s election victory, sent lawmakers into hiding for their own safety and left five people dead including a police officer.

A failed bid last month to dismiss the case against Trump on the basis that it would be unconstitutional to hold a post-presidency trial drew the support of 45 of the 50 Republicans in the Senate. To secure a conviction, 17 Republicans would need to join the Senate’s 50 Democrats in the vote, a daunting hurdle.

Trump’s first impeachment trial, on charges of abuse of power and obstructing Congress arising from his request that Ukraine investigate Biden and his son Hunter, ended last year in acquittal by the then-Republican-led Senate.

Trump parted ways with his initial legal team amid a reported dispute over how to respond to the impeachment charge.