TOLUCA, Mexico—The Mexican unit of Spanish construction giant Obrascón Huarte Laín SA is grappling with questions about its accounting and ties to the administration of Mexican President Enrique Peña Nieto.

At the center of both are a series of lucrative contracts that OHL de Mexico SAB won or which were extended under Mr. Peña Nieto’s term as governor of the State of Mexico between 2005 and 2011.

The contracts include building and operating a toll road outside Mexico City whose estimated cost has soared to about $3.6 billion. Critics say it could become Mexico’s most expensive highway.

Mexican securities regulators in October alleged the firm applied improper accounting practices related to the toll road that boosted income and asset valuations. The parent company in Madrid has denied wrongdoing and said its accounting was approved by auditors and Spanish financial regulators.

OHL’s share price has fallen 73% since April 30 on concerns about further fallout from the Mexican project and a steeply discounted share-rights offering. OHL’s global concession division, which includes the toll project, generated 80% of the firm’s pretax profit and contributed 12.5% of OHL’s €3.73 billion in revenue last year.