By Burcu Karakas and Huseyin Hayatsever
ANKARA (Reuters) - 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.
Ogan, 55, a former academic, was the first-round presidential candidate of an alliance of right-wing parties led by the Victory Party, which is known for its anti-immigrant stance in Turkey, the world's biggest host of refugees.
In an interview with Reuters last week, Ogan said his goal was to remove two mainly Kurdish parties from Turkey's "political equation" and bolster Turkish nationalists and secularists.
The pro-Kurdish party HDP has endorsed Kilicdaroglu, while the Kurdish-Islamist Huda-Par backs Erdogan.
Kilicdaroglu has pledged to roll back much of Erdogan's sweeping changes to Turkish domestic, foreign and economic policies, including reversing an unorthodox economic programme to address a cost-of-living crisis.
Erdogan has said a vote for him in the runoff is a vote for stability.
In an interview with state broadcaster TRT late on Monday, Erdogan expressed his pleasure at Ogan's endorsement.
"I believe that this union of forces will be beneficial for our country and nation," Erdogan said, adding that he and Ogan were in agreement on many issues including the fight against terrorism as well as relations with Turkic states.
Commenting on Ogan's anti-immigrant stance, Erdogan said his government already had plans to re-settle 1 million refugees in Syria, and a timetable for that plan could be discussed in talks with the Syrian government after the runoff.
Analysts say Ogan's support should give Erdogan a boost but also divide Ogan's supporters. The Victory Party will separately announce its own stance on the runoff on Tuesday.
Erdogan's strong showing in the initial vote confounded pollsters who had said Kilicdaroglu led opinion polls. They later pointed to an unexpected surge in nationalist support at the ballot box to explain the result.
Last week, Kilicdaroglu, head of the secularist Republican People's Party (CHP) and candidate of a six-party alliance, sharpened his tone and vowed to send all migrants back to their countries once elected.
A small member of Ogan's alliance, the Justice Party, quit the bloc at the weekend and endorsed Kilicdaroglu in the runoff.
One Ogan supporter said last week she would not vote in the runoff because the remaining two candidates were not appealing.
"I voted for Ogan in the first round, but I am not planning to vote in the second round. My heart and my mind say 'No' to both candidates who aligned with terrorist organizations," Fidan, 33, who lives in Germany, said last week.
Ogan entered parliament in 2011 with the nationalist MHP, launched an unsuccessful bid in 2015 for the party's leadership, and was later expelled.
(This story has been corrected to fix spellings of party abbreviation HDP and names of Erdogan and Ogan in paragraphs 8-20)
(Reporting by Burcu Karakas, Huseyin Hayatsever and Ece Toksabay; writing by Jonathan Spicer; editing by Tom Perry, William Maclean and Mark Heinrich)