The third-placed candidate in Turkey's election Sinan Ogan on Monday announced his support for President Recep Tayyip Erdogan in Sunday's electoral run-off.
Mr Ogan endorsed Mr Erdogan at a news conference in Ankara and said his campaign made Turkish nationalists “key players” in politics.
He said he had met representatives of both Mr Erdogan and challenger Kemal Kilicdaroglu before making the decision, Turkey’s Posta newspaper reported.
Mr Ogan's right-wing Ancestral Alliance gained 5.2 per cent of the vote in the tight election and his backing has been seen as crucial in propelling either Mr Erdogan or Mr Kilicdaroglu over the line in the run-off vote.
Mr Erdogan received 49.5 per cent of the ballot on May 14 compared to Mr Kilicdaroglu's 44.9 per cent, while the ruling party's coalition won a majority in parliament, giving the President an advantage as he seeks to extend his two-decade rule.
Mr Ogan, 55, a former academic, led an alliance of right-wing parties headed by the Victory Party, known for its anti-immigrant stance, in the first round of the presidential elections.
“As a result of these negotiations and consultations and messages from the grass roots, I declare that we will support the President of the People's Alliance candidate [Mr Erdogan] in the second round,” he said.
“It is useful to know that we have taken this decision after all kinds of consultations. I hope that the decision we have made, without any personal expectations, will be beneficial to the great Turkish nation.”
Mr Kilicdaroglu has pledged to reverse many of Mr Erdogan's sweeping changes to Turkish domestic, foreign and economic policies, including an unorthodox economic programme to address a cost-of-living crisis.
Seen as a bid to win Mr Ogan's backing, Mr Kilicdaroglu came out heavily against refugees and immigrants, vowing to deport thousands of Syrians if he wins.
Mr Erdogan has said a vote for him in the run-off is a vote for stability.
Analysts say Mr Ogan's backing should give Mr Erdogan a boost but also divide Mr Ogan's supporters.
The Victory Party will separately announce its own stance on the run-off on Tuesday.
Alper Coskun, a senior fellow at the Carnegie Endowment for International Peace and veteran Turkish diplomat, has said Mr Erdogan is heading to the run-offs with an advantage.
He said: "The alliance that he has been able to put together with ultraconservative and extreme political parties has been able to win a majority in parliament."
At more than 320 of the 600 total seats in parliament, Mr Erdogan has a "strong moral advantage over the opposition".
However, Mr Coskun said Mr Ogan's supporters would not necessarily be loyal.
"He seems to be supported by disgruntled voters who neither want to support Erdogan nor believe that Kilicdaroglu is the right person to lead them," he said.
"It is dubious whether [Mr Ogan] holds full control over those who have supported him. So his endorsement .... may not necessarily translate to an increase of five percentage points", for the person he backs, Mr Coskun said.
Mr Ogan earlier said he would only back a candidate who cracks down on migrants and fights "terrorism" – a reference to Kurdish militants, the Kurdistan Workers’ Party, or PKK, which Turkey has designated a terrorist group.
Hamish Kinnear of the Verisk Maplecroft consultancy believes Mr Erdogan does not need a big chunk of voters for Mr Ogan to win the presidency.
"Assuming Erdogan's first-round voters remain on side, only a small portion of Ogan's voters need to go with Erdogan to push the President into his third decade in power," he told AFP.
