Lawyers for the pornographic film star Stephanie Clifford filed a defamation lawsuit against President Trump on Monday, based on statements he made on Twitter two weeks ago that questioned her credibility.

Ms. Clifford, known professionally as Stormy Daniels, released a sketch on April 17 of a man she said had threatened her in 2011. She said the man had approached her in Las Vegas, telling her to “leave Trump alone.”

Mr. Trump’s advisers had encouraged him to stay silent about Ms. Clifford, who has said she had an affair with the future president in 2006. For many months, he heeded the advice. But he finally spoke up after the sketch was released, tweeting that it was “a total con job.”