Former president Donald Trump's lawyers declined to comply with a request from House Democrats on Thursday that he give evidence under oath in the Senate impeachment trial next week.
Lawyers Bruce Castor and David Schoen called the request by House impeachment managers a "public relations stunt" and the entire trial an "unconstitutional proceeding".
Mr Trump is being represented by Mr Castor and Mr Schoen after two lawyers withdrew from defending him.
"Your letter only confirms what is known to everyone: you cannot prove your allegations against the 45th President of the United States, who is now a private citizen," they said.
The impeachment managers, referring to a pre-trial brief filed on Tuesday, asked Mr Trump to address his legal team's denial of claims made in the article of impeachment concerning the events of January 6, when a deadly pro-Trump riot occurred at the US Capitol.
"Two days ago, you filed an answer in which you denied many factual allegations set forth in the article of impeachment," read the letter from Jamie Raskin, a Democrat from Maryland.
Mr Raskin is lead impeachment manager on a team of nine Democratic representatives who have the task of organising the charge against Mr Trump when the Senate trial begins on February 9.
"You have thus attempted to put critical facts at issue, notwithstanding the clear and overwhelming evidence of your constitutional offence," he wrote.
"The use of our constitution to bring a purported impeachment proceeding is much too serious to try to play these games," Mr Castor and Mr Schoen responded.
It is not yet clear how the Democratic impeachment managers will move forward – holding the trial without Mr Trump or voting to subpoena him.
The House charged the former president with "incitement of insurrection" over the violent events of January 6, when a mob stormed the legislative building after Mr Trump gave a speech outside the White House.
Five people died in the riot, including a US Capitol Police officer.
In the pre-trial brief, the former president's legal team claimed that Mr Trump had not incited the crowd of his supporters to riot.
"In light of your disputing these factual allegations, I write to invite you to provide testimony under oath, either before or during the Senate impeachment trial, concerning your conduct on January 6, 2021," the letter from Mr Raskin says.
The team asked that Mr Trump give evidence as early as February 8, before the trial starts, or as late as February 11.
"If you decline this invitation, we reserve any and all rights, including the right to establish at trial that your refusal to testify supports a strong adverse inference regarding your actions [and inaction] on January 6, 2021," Mr Raskin wrote.
Mr Castor and Mr Schoen responded to that assertion, declaring "there is no such thing as a negative interference in this unconstitutional proceeding".
For a conviction to pass, at least two thirds of the Senate has to vote for it, meaning 67 senators are needed to convict.
This may be a challenge, with low support in the Republican Party for the trial.
The Senate is split 50-50 between Republican and Democrat senators, with Vice President Kamala Harris casting the deciding vote.