It is possible for presidents and politicians to be good Catholic citizens and it is possible for them to become saints.
There are a number of saints that they can use as their example. St. Casimir, beloved of both Lithuanians and Poles. He was often found kneeling in prayer outside of closed churches. St. Louis IX, the only king of France to be canonized. St. Isabel of Portugal, married to the King of Portugal. Elizabeth was a Third Order Franciscan. These are just three of the many men and women who severed their nations, but served God first.
his great video of the new president of Poland chasing after a host that had flown away during an outdoor Mass. If only all Presidents and politicians had such a love for Our Lord in the Blessed Sacrament and His Holy Church. H/t to Fr. Z