The CHP was created by modern Turkey's founder, Mustafa Kemal Ataturk, and is the country's oldest political party, though it has been out of power centrally since the 1990s.
However, Mr Kilicdaroglu has broadened its appeal by embracing minority groups and has formed alliances with right-wing parties.
He has also shown himself willing to challenge Mr Erdogan, a leader who has become increasingly intolerant of criticism.
In February's earthquake, in which more than 45,000 people were killed in south-eastern Turkey, Mr Kilicdaroglu led attacks on the government, which he said had allowed corruption and poor building standards.