When did the campaign of political  repression that Mikhail Demichev was arrested during occur?
Great Purge The Great Purge or the Great Terror (Russian: Большо́й терро́р ) was a campaign of political repression in the Soviet Union which occurred from 1936 to 1938. It involved a large-scale purge of the Communist Party and government officials, repression of peasants and the Red Army leadership, widespread police surveillance, suspicion of "saboteurs", "counter-revolutionaries", imprisonment, and arbitrary executions. Mobile gas vans were invented to execute people without trial. In Russian historiography, the period of the most intense purge, 1937–1938, is called Yezhovshchina (Russian: Ежовщина ; literally, "Yezhov phenomenon", commonly translated as "times of Yezhov" or "doings of Yezhov"), after Nikolai Yezhov, the head of the Soviet secret police, the NKVD, who was later killed in the purge. It has been estimated that 600,000 people died at the hands of the Soviet government during the Purge.
Polish Operation of the NKVD The Polish Operation of the Soviet NKVD security service in 1937–1938 was a mass operation of the NKVD carried out against purported Polish agents in the Soviet Union during the period of the Great Purge. It was ordered by the Politburo against the so-called "Polish spies" and customarily interpreted by the NKVD officials as relating to "absolutely all Poles". It resulted in the sentencing of 139,835 people, and summary executions of 111,091 ethnic Poles, as well as those accused of working for Poland. The operation was implemented according to NKVD Order № 00485 signed by Nikolai Yezhov. The majority of the victims were ethnically Polish but not all, according to Timothy Snyder. The remainder were 'suspected' of being Polish, without further inquiry. In order to speed up the process the NKVD personnel reviewed local telephone books and arrested persons with Polish-sounding names.