Deniz Baykal was the leader of the secularist Republican People's Party (CHP), which has threatened to try to block constitutional reforms proposed by Prime Minister Tayyip Erdogan's ruling AK Party.
"I will not give up in the face of such a black campaign. I will not let anybody question me," Baykal told a televised news conference.
Baykal says the AK Party, whose roots lie in political Islam, must have had knowledge of the videotape.
"If this has a price, and that price is the resignation from CHP leadership, I am ready to pay it. My resignation does not mean running away, or giving in," Baykal said. "On the contrary, it means that I'm fighting it." The Turkish government is preparing to send a constitutional reform package to a national referendum, but the CHP has threatened to take the issue to the Constitutional Court.
The videotape was released on YouTube late last week.
The 71-year-old Baykal led the CHP since 1992. Critics say Baykal's domination of the CHP had stifled chances for younger politicians who could have made the party more progressive and appealing to Turkish voters.
The country's next election is due by July 2011, when the AK Party will seek a third term in power.
