Erdogan faces the fight of his political life in what many view as Turkey's most consequential election since its birth as a post-Ottoman republic 100 years ago.
The 68-year-old leader needs to overcome the twin hurdles of an economic crisis and that aftermath of a devastating earthquake as he seeks to extend his Islamic style of rule until 2028.
Opinion polls point to a tight race that remains far too close to call.
But Erdogan's task had appeared to become a little easier when one of the main leaders of the six-party opposition alliance walked out of the talks on Friday.
'The nation won'
Meral Aksener has argued that Kilicdaroglu -- a soft-spoken 74-year-old from Turkey's long-marginalised Alevi community -- lacked the public appeal to defeat Erdogan in the presidential ballot.
She has instead urged the popular CHP mayors of either Istanbul or Ankara to step into the race.
The two met Aksener on Monday in a make-or-break attempt to bring her nationalist Iyi Party back into the opposition fold.
"Our nation cannot tolerate a split," Ankara mayor Mansur Yavas told reporters after the meeting.
The opposition last united in an effort to unseat Erdogan's allies in municipal elections held in 2019.
A 12-point text hammered out Monday committed Kilicdaroglu to naming the other five party leaders as vice president for a post-election transition period should he win.
Kilicdaroglu would then appoint the two mayors as vice president "at a time deemed appropriate by the president".
Party leaders said Aksener made the two appointments her main condition for rejoining the opposition bloc.
"The nation won," Iyi Party deputy leader Bahadir Erdem tweeted after the announcements.
Analysts viewed the opposition's failure to put aside their differences just two months before the vote as one of the main factors working in Erdogan's favour.
Erdogan's public approval plunged after he unleashed an unusual economic experiment in late 2021 that tried to fight inflation by drastically cutting interest rates.
A resulting currency crisis erased people's savings and pushed the annual inflation rate to an eye-watering 85 percent.
Turkish stocks and eurobonds both rallied on rising investor hopes that the joint candidate will be able to beat Erdogan and restore economic orthodoxy after years of tumult.
Kilicdaroglu is now also expected to hold separate talks with the pro-Kurdish HDP -- parliament's third-largest party -- to see if he can further expand his appeal.
The HDP had been excluded from the opposition talks because of Aksener's more hawkish policy views.
"We embrace the people's desire for change," HDP co-leader Mithat Sancar said after Kilicdaroglu's name was announced.
"The HDP will do its part."
Erdogan seeks forgiveness
Erdogan's widely praised handling of Russia's invasion of Ukraine helped reverse the slide in his approval rating and set him up with a chance to secure a come-from-behind win.
But a catastrophic earthquake last month that killed more than 46,000 people in Turkey and nearly 6,000 in Syria threatened to doom Erdogan's entire political career.
Erdogan conceded that his government had been slow to respond in the first critical days of the crisis and asked voters to forgive him for rescue delays.
He also brushed aside rumours that he would try to delay the May vote to a more politically favourable date.
"We are not hiding behind excuses," he said last week.
He told a weekly cabinet that he will announce the formal start of the election campaign on Friday.