Former President Donald Trump's lawyers on Thursday rejected a request from Democrats to testify at his impeachment trial in the U.S. Senate next week, dismissing their invitation as a "public relations stunt."

Democrats in the House of Representatives accuse Trump of inciting insurrection when he urged supporters to "fight" his election defeat before they stormed the Capitol on January 6, fought with police and sent lawmakers scrambling for safety.

Read more:

U.S. Capitol riots: Five died

Biden and several ex-presidents condemn U.S. Capitol riots

Five people died, including a Capitol police officer.

"The president will not testify in an unconstitutional proceeding," Trump adviser Jason Miller told Reuters. In an open letter, Trump's attorneys, Bruce Castor and David Schoen, called the request a "public relations stunt."

The attorneys this week rejected the impeachment charge and asserted his claims that his election defeat was the result of widespread fraud – which were baseless – were protected by the First Amendment of the U.S. Constitution.

Democratic lawmaker Jamie Raskin, the lead House impeachment manager, wrote in a letter to the Republican Trump and his attorney inviting the former president, who left office on January 20, to provide testimony under oath.

"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," Raskin wrote.

In their response, Trump's lawyers wrote to Raskin: "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."

Several senators said it would have been a bad idea for Trump to testify. Republican Senator Lindsey Graham, a vocal Trump ally, told reporters: "I don't think that would be in anybody's interest."

For two months after losing his re-election bid to President Joe Biden, Trump loudly argued that he lost due to rampant electoral fraud, claims that were rejected by multiple courts and state election officials.

Trump's lawyers and most Republican senators have challenged the constitutionality of the trial. They have said the Senate does not have the authority to hear the case because Trump, also a Republican, has already left office and cannot be removed.

Such an argument would allow Republican senators – who hold half the seats in the chamber – to vote against Trump's conviction on procedural concerns instead of directly supporting his comments.

A two-thirds majority of the 100-member Senate would have to support the charge to convict Trump, meaning 17 Republicans would need to join all 50 Democrats in backing it.