Thanks to the country's complicated electoral rules governing the two-vote system, the total number of seats in the Bundestag, already hefty at 672, keeps on growing.
Despite the emergence of the centrist contender Francois Bayrou as a third force at the presidential election, the parliamentary vote largely confirmed the two-party system.
Mr Stephen is widely regarded to be the front runner in the two-horse race, but with the new leader to be elected under a one-member-one-vote system, Mr Rumbles said there is "all to play for".