Erdogan, who has transformed Turkey since 2003 as prime minister and now president, likes to see himself as the undisputed heavyweight champion of Turkish elections and undefeated at the ballot box.
But analysts say his ruling Justice and Development Party (AKP) could lose its overall majority in the June 24 simultaneous presidential and parliamentary polls to an opposition alliance.
While Erdogan is favourite to win a second presidential mandate with the enhanced powers agreed in last year's referendum, the election could go to a run-off where victory for the Turkish strongman would not be a foregone conclusion.
The main opposition Republican People's Party (CHP) leader Kemal Kilicdaroglu, a bookish figure who rarely rattled Erdogan, shook up the poll by choosing not to run himself and giving the nod to Muharrem Ince, a fiery MP from the party's left.
While Ince has energetically criss-crossed Turkey and happily ripped out pages from the Erdogan guide to no-holds-barred campaigning, his party has also forged a broad alliance with the dissident nationalists of former minister Meral Aksener and the conservative Saadet (Felicity) Party.
"The opposition is showing a certain degree of coordination and unity for the first time," Asli Aydintasbas, fellow with the European Council on Foreign Relations (ECFR) told AFP, saying it could even "gain the upper hand" in parliament.
She said the AKP had underestimated Ince after Kilicdaroglu who was considered an "easy" opponent.
"Now Erdogan is faced with someone who talks back, much like he does, and is being listened to."
'Opposition capitalising'
The elections are also taking place with a troubled economy looming in the background instead of being the usual campaign ace for Erdogan.
Inflation has spiked up to 12.15 percent and the lira has lost some 20 percent in value against the dollar this year.
The president is facing a "combination of a difficult economic context and a surprisingly vigorous and unified opposition," Paul T. Levin, director of the Stockholm University Institute for Turkish Studies, told AFP.
Ince was displaying "his strong rhetorical abilities and taking the fight to Erdogan", said Levin.
Levin added there was also growing dissatisfaction over the presence of 3.5 million Syrian refugees in Turkey. "The opposition are capitalising on this."
Erdogan barely needed to break into a sweat to finish off his opponent in the August 2014 presidential elections, the softly spoken Ekmeleddin Ihsanoglu who, ironically, has said he will vote for his former rival in these polls.
But Ince has been unafraid to touch nerves, playing up the past cooperation between the AKP and the US-based preacher Fethullah Gulen who is accused of being behind the 2016 failed coup.
He even alleged that Erdogan had met Gulen at his Pennsylvania compound to win blessing for forming the AKP. Erdogan filed a criminal complaint against Ince over the "baseless" claim.
Touting himself as a candidate of reconciliation, Ince has also sought to make headway with voters who are not natural supporters of the CHP, such as Turkey's Kurds and religious conservatives.
The opposition are "setting the agenda for the first time in many elections," said Aydintasbas at ECFR.
'First time in long while'
Ince supporters even see the surprise landslide victory of businessman Ali Koc who ousted longstanding boss Aziz Yildirim on May 3 as chairman of Istanbul football giants Fenerbahce as a precursor of things to come.
"The Fenerbahce community realised change; now what Turkey needs is a big change," Ince tweeted.
In contrast to the opposition alliance, Erdogan's pact with the Nationalist Movement Party (MHP) is faring less well, with its leader Devlet Bahceli near invisible in the campaign so far.
After a conspicuously low-key start, Erdogan has stepped up his campaign appearances in recent days and his face now stares down at Turks from billboards nationwide proclaiming "a great Turkey wants a strong leader".
The spotlight will be on fair play in the elections, with the OSCE deploying a mission of 12 staff in Ankara and 22 long-term observers across the country to monitor the conduct of the polls.
Over a fortnight of intense campaigning remains and Erdogan has markedly stepped up his rhetoric against Kurdish militant leaders in northern Iraq, indicating he may seek to impress voters with a last minute military success.
Marc Pierini, former EU ambassador to Turkey and visiting scholar at Carnegie Europe, said it would be "imprudent" to make predictions on elections that were an "all-out fight".
But he added: "For the first time in a long while, opposition parties have a shot at putting forward a radically different option for voters."
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