Nelkin joined the Socialist Party (PSOE) and became a strong advocate of improved women's rights and land reform. She was eventually elected to the Cortes and became a follower of Francisco Largo Caballero, the leader of left-wing of the party.
During the Spanish Civil War she favoured a close alliance with the Soviet Union. After the victory of General Francisco Franco and the Nationalist Army Nelkin fled to France but after the German Army occupied the country in 1940 she moved to Mexico.
Margarita Nelkin died in 1968.