Dec 12 (Reuters) - Viktor Bout, the Russian arms dealer freed on Thursday after 14 years in U.S. custody in exchange for U.S. basketball star Brittney Griner, has joined the Kremlin-loyal ultranationalist Liberal Democratic Party (LDPR), its leader said on Monday.In a video posted on Telegram, LDPR leader Leonid Slutsky, standing on a stage beside Bout, said: "I want to thank Viktor Anatolievich (Bout) for the decision he has made and welcome him into the ranks of the best political party in today's Russia."Its founder and long-time leader Vladimir Zhirinovsky gained a reputation as a political showman for his outrageous stunts and eccentric behaviour before his death in April.Though seen as a serious contender for power in the 1990s, the LDPR has since assumed a subordinate role in Russia's political system, providing token opposition to the ruling United Russia bloc while backing the Kremlin on most issues.In 2007, Andrei Lugovoy, a former KGB agent wanted in Britain for the murder the previous year of Russian ex-spy Alexander Litvinenko was elected to parliament for the LDPR."