Turkish President Tayyip Erdogan intensified pressure on the opposition candidate in a re-run election for mayor of Istanbul by saying he would be barred from taking office if found guilty of insulting a provincial governor.

Ekrem Imamoglu, from the main opposition Republican People’s Party (CHP), narrowly beat the candidate of Erdogan’s AK Party in a March mayoral contest. After AKP appeals, the election commission annulled the result and ordered a re-run on June 23.

The AKP’s loss of Istanbul in the March 31 local elections was one of the worst setbacks for Erdogan since his Islamist-rooted party swept to national power in 2002. The AKP also lost control of the capital Ankara.

After keeping quiet on the mayoral race in recent weeks, Erdogan accused Imamoglu of being in cahoots with the network of Fethullah Gulen, the U.S.-based Muslim cleric accused by Turkish authorities of masterminding a failed military coup in July 2016.

Erdogan also said Imamoglu would face consequences for allegedly insulting the governor of Ordu, a Black Sea province north of Istanbul, while campaigning there.

In a radio show on Wednesday, Erdogan said the governor, Seddar Yavuz, was set to take the matter to court.

“I cannot know right now what decision the judiciary will make. But the decision by the judiciary could block (Imamoglu’s) path in this (election),” he said.

Turkish media said Imamoglu got into a row earlier this month at Ordu airport after being barred from using the VIP lounge. AKP supporters said that in a heated exchange with airport officials Imamoglu called Yavuz “a dog”. Imamoglu denied this, saying he used a different word that sounded similar.

“This is a plot to pull the arguments on this issue to another level. He’s not the governor of the state, but rather of the ruling party, and that is how this should be seen,” CHP leader Kemal Kilicdaroglu said on Wednesday.

Erdogan previously said Imamoglu could not serve as mayor unless he apologised for the alleged insult, but Kilicdaroglu rejected this position, saying Istanbul’s mayor would be elected by the people, not appointed by Erdogan.

The decision to re-run the Istanbul mayoral election has drawn international ire and accusations from Turkey’s opposition that Erdogan is gradually dismantling democracy.

It has also unnerved financial markets and thrown a spotlight on the AKP’s management of Turkey’s largest city and commercial hub during its long years in power.