I won't say that defeating the top man in Russia in democratic elections is impossible. But...there are reasons it's never, ever happened.
Indeed, there have been some who questioned if Yeltsin really won in 1996:
"The phrase that Medvedev uttered in response "will go down in history," said Sergei Babkin, the leader of an opposition party, who was the first to reveal the details of the closed-door meeting during a radio interview the following day. "He brought up the presidential elections of 1996 and said, 'There is hardly any doubt who won [that race]. It was not Boris Nikolaevich Yeltsin."
http://content.time.com/time/world/article/0,8599,2107565,00.html