Already reeling from corruption scandals and a declining security situation, President Enrique Peña Nieto of Mexico was accused on Sunday of plagiarizing nearly a third of his law school thesis, according to a report by an investigative journalist.

The journalist, Carmen Aristegui, has reported on several articles that have contributed to the president’s declining public approval ratings, including the disclosure in 2014 of his wife’s questionable real estate dealings with major government contractors.

Image Carmen Aristegui, an investigative journalist, has reported on several articles that have contributed to the president’s declining public approval ratings. Credit... Henry Romero/Reuters

Ms. Aristegui lost her job in 2015 after her reporting on the Mexico City mansion built for the president’s wife, Angélica Rivera, by a firm that had received hundreds of millions of dollars in government contracts. Since then, Ms. Aristegui has been publishing her work on her own website.