Stephanie Clifford, the pornographic film star better known as Stormy Daniels, was scheduled to meet on Monday for the first time with prosecutors investigating Michael D. Cohen, President Trump’s longtime personal fixer, but the meeting was canceled, according to tweets from Michael Avenatti, Ms. Clifford’s lawyer.

For several months, prosecutors from the United States attorney’s office in Manhattan have been looking into whether Mr. Cohen broke the law in any of his various business dealings — among them, a $130,000 payment he made to Ms. Clifford in the run-up to the 2016 election that was meant to keep her quiet about an extramarital affair she claims she had with Mr. Trump.

Ms. Clifford has been cooperating with the prosecutors, including providing them with documents about the payment, in response to a subpoena, a person familiar with the case said. But the interview on Monday would have given prosecutors their first opportunity to ask Ms. Clifford detailed questions about the arrangement, which Mr. Trump initially denied but then acknowledged in a startling turnabout last month.

It was unclear exactly what Ms. Clifford had planned to tell the prosecutors. But legal experts have said that if the payment to her was intended to influence the presidential race by suppressing an explosive news story that could have damaged Mr. Trump’s election chances, it could be covered by federal campaign finance laws.