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 hardline 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 told a press conference in Ankara, adding his campaign had made Turkish nationalists "key players" in politics.

Kilicdaroglu's Nation Alliance "failed to convince us about the future," while the decision to back Erdogan was based on a principle of "non-stop struggle (against) terrorism," he said.

Erdogan received 49.5% support on May 14 compared to Kilicdaroglu at 44.9%, while the ruling party's coalition won a majority in parliament. That gives Erdogan an advantage as he seeks to extend his two-decade rule in what is one of Turkey's most consequential elections ever.

Image - Reuters