CHIHUAHUA, Mexico — A former deputy in President Enrique Peña Nieto’s party has been arrested as part of an investigation into the illegal use of public money to fuel his party’s campaigns in Mexican elections last year, state officials announced on Wednesday.

The former deputy, Alejandro Gutiérrez, is one of several allies of Mexico’s president suspected of taking part in an embezzlement scheme to propel his party’s chances at a time of deep dissatisfaction with the government, according to former officials connected to the scheme and hundreds of pages of documents reviewed by The New York Times.

The arrest of Mr. Gutiérrez, a veteran figure who helped control the finances of the governing Institutional Revolutionary Party, or PRI, is part of a widening inquiry into the enormous corruption that occurred under the previous governor of the state of Chihuahua. The governor was a close friend of Mexico’s president and a rising star in the party before he fled to the United States to avoid corruption charges.

The case against him has already ensnared more than a dozen former state officials, some of whom are now cooperating with the authorities.