Critics say the investigation into Moscow Communist Party first secretary Valery Rashkin is politically motivated, after he led protests in the Russian capital following parliamentary elections in September.
Rashkin and other Communists accused the ruling United Russia party of fraud after it won an overwhelming majority in the election.
During a plenary session on Thursday, a majority of lawmakers from Russia's lower house State Duma backed removing Rashkin's immunity as requested by prosecutors.
The poaching investigation was launched in October after Rashkin and a friend were found with parts of an elk carcass in the trunk of a car near the Volga city of Saratov.
Rashkin, 66, initially denied any illegal hunting but later admitted he had killed the animal, claiming he had mistaken it for a wild boar.
He proposed paying 80,000 rubles ($1,070, 950 euros) in compensation for the killing, or to buy a cow elk and release it into the woods.
With nearly all vocal Kremlin critics barred from running in the September elections, many Russians backed the Communists as a form of protest voting.
The party, the successor to the one that ruled the Soviet Union, came second with about 19 percent of the vote, trailing President Vladimir Putin's United Russia, which took nearly 50 percent.
The past year has seen an unprecedented crackdown on critical voices in Russia, with main opposition politician Alexei Navalny imprisoned in February for two-and-a-half years on old fraud charges.
His political organisations were banned, while a number of independent media and Kremlin critics were designated "foreign agents".