Turkey's third-place election candidate endorsed President Tayyip Erdogan on Monday, boosting the incumbent and intensifying the challenges for opposition candidate Kemal Kilicdaroglu in a Sunday runoff vote.

Sinan Ogan, a hard-line nationalist who was little known among the broader public before the campaign, won 5.2% support in the initial presidential election on May 14, prompting some analysts to call him a potential "kingmaker" for the runoff.

"I declare that we will support the People's Alliance candidate Mr. Recep Tayyip Erdogan in the second round," Ogan said during a news conference in Ankara, adding his campaign had made Turkish nationalists "key players" in politics.