Lawyers for US President Donald Trump are seeking $20 million (£14m) in damages from a porn actress he says broke a non-disclosure deal signed before the 2016 elections.
They argue Stormy Daniels breached the agreement at least 20 times.
The actress claims she had an affair with Mr Trump that began in 2006 and lasted several months.
The president's lawyer, Michael Cohen, has said he paid her $130,000 of his own money to remain silent.
This is the first time President Trump has become directly involved in the case. He "vehemently denies" the alleged affair.
The lawyer acting for Stormy Daniels, whose real name is Stephanie Clifford, this month filed a lawsuit aiming to scrap the non-disclosure agreement.
Michael Avenatti said the filing by the law firm representing the president were "unprecedented" and "yet another bullying tactic" against her.
The filing also sought a change of venue from state to federal court.