From the Wikipedia page [1]
The Mordovian Autonomous Soviet Socialist Republic (Russian: Мордовская Автономная Советская Социалистическая Республика, Mordovskaya Avtonomnaya Sovetskaya Sotsialisticheskaya Respublika; Erzya: Мордовскяй Автономнай Советскяй Социалистическяй Республикась, Mordovskjaj Avtonomnaj Sovetskjaj Socialističeskjaj Respublikaś; Moksha: Мордовской Автономной Советской Социалистической Республикась, Mordovskoj Avtonomnoj Sovetskoj Socialističeskoj Respublikaś) was an autonomous republic of the Russian SFSR within Soviet Union now known as the Republic of Mordovia.
The Mordovian Autonomous Soviet Socialist Republic was established on December 20, 1934 after the transformation of Mordovian Autonomous Oblast in Kuybyshev Krai. After Kuybyshev Krai was itself transformed into Kuybyshev Oblast, the Mordovian ASSR was separated from it and subordinated directly to the Russian Soviet Federative Socialist Republic.
On December 7, 1990, the Mordovian ASSR adopted the Declaration on the legal status of the Republic of Mordovia, which changed the status of the republic from ASSR to an SSR. In 1990, shortly before the dissolution of the Soviet Union, the republic became the Republic of Mordovia. In its present form, the republic has existed since January 25, 1994.