Istanbulites are returning to the polls Sunday after the mayoral election in March was annulled over alleged irregularities.
Local elections around Turkey on March 31 showed the ruling Justice and Development Party (AKP) of President Recep Tayyip Erdogan remained the most popular overall, but it suffered a shock defeat in Istanbul, as well as losing the capital Ankara.
The AKP disputed the result and the top election body accepted claims of fraud and called a re-run for June 23.
It did not annul votes for city council seats, where the majority went to Erdogan's party.
The AKP's Binali Yildirim, an Erdogan loyalist and former premier, lost by around 13,000 votes to Ekrem Imamoglu of the Republican People's Party (CHP) in March.
After two weeks of multiple recounts, the AKP applied in April to the Supreme Electoral Council (YSK) to annul the vote, claiming there was widespread corruption and theft at the ballot box.
There were claims some of the ballot box staff were not civil servants as required by law.
Critics of the ruling party claim the AKP pressured election authorities into calling the re-run because Istanbul is Turkey's economic powerhouse, home to 15 million people, and vital to the party's political machine.
"The municipality spends billions of US dollars on public tenders and services, which puts the AKP in direct contact with voters. In short, it's the gasoline on which the AKP machine runs," said Berk Esen, assistant professor of international relations at Ankara's Bilkent University.
But Abdullah Guler, an AKP lawmaker in Istanbul, dismissed the allegations.
"If the AKP looked at the situation like this, it would have done the same (in other big cities like Ankara and Antalya)," he told AFP.
Erdogan has been more prominent in the last stretch of the June campaign, but nothing compared to the 102 rallies he held in just 50 days across the country ahead of the March election.
Guler said less visibility was normal for a single-city election.
But others, like Esen, believe Erdogan is avoiding a major presence "so that he would not be the face of defeat, which seems very likely according to the opinion polls.
"He is a polarising figure and they are trying a reconciliatory strategy," Esen said.
The results could damage Erdogan's reputation for invincibility -- his party has won every election since taking power in 2002 -- and embolden rivals within the AKP such as ex-premier Ahmet Davutoglu or former economy minister Ali Babacan, who are rumoured could set up new parties.
Analysts say Erdogan faces "a lose-lose situation": a defeat would trigger "major chaos" inside the party; a victory would be greeted with suspicion by critics at home and abroad, where the election annulment has already drawn criticism.
"The whole democratic world will lose further faith in the process and in Turkey," said Ayse Ayata, a professor at Ankara's Middle East Technical University.
Investors are already concerned over Turkey's trajectory amid slowing growth and a rift with Washington over the purchase of a Russian missile defence system that could trigger US sanctions.
Guler, the AKP lawmaker, denied there was any risk to Erdogan or the party.
"The AK Party continues to have the support of the public," he said, pointing to its control of 760 of Turkey's 1,389 local authorities.