López was a member of the Constituent Assembly of year XIII, representing Buenos Aires. At the request of the Assembly, he wrote the lyrics to a "patriotic march", which eventually became the Argentine National Anthem. It was a military march, whose music was composed by the Catalan Blas Parera; it was approved on March 11, 1813. The first public reading was at a tertulia on May 7 in the house of Mariquita Sánchez de Thompson. It displaced a different march, written by Esteban de Luca, which would have been the anthem if not for the more militaristic Lopez.