sentences:  The close relationship between the PRI and the Mexican state has been examined by a number of scholars.Even though the armed phase of the Mexican Revolution had ended in 1920, Mexico continued to encounter political unrest.A grave political crisis caused by the July 1928 assassination of president-elect Álvaro Obregón led to the founding on 4 March 1929 of the National Revolutionary Party (, PNR) by Plutarco Elías Calles, Mexico's president from 1924 to 1928.