Mr. Erdogan and Mr. Arinc linked the corruption investigation to the Gezi protests, suggesting they were part of one conspiracy against the government and the governing party. Mr. Erdogan emerged from the protests with his popularity intact among his religiously conservative base, for the most part. “This is likely to lead to divisions within his own constituency, which is a greater threat to him,” said Sinan Ulgen, a visiting scholar at Carnegie Europe who runs his own economic and foreign policy think tank in Istanbul.

Image Fethullah Gulen Credit... Ruth Fremson/The New York Times

The allegations of corruption center on the construction industry and urban development projects in Istanbul, an important source of money and power for businessmen affiliated with the Justice and Development Party. They present a difficult challenge to Mr. Erdogan because fighting corruption was an important pillar of his party’s rise.

“Part of the image building of the earlier years of the party was that it would be a clean break with the corrupt Turkish politics of the past,” Mr. Ulgen said.

The tale of corruption, which has unfolded through a series of press leaks, is familiar to many Turks, with echoes of the tactics employed in recent years as top generals and military officers were sent to prison on charges of plotting coups. Those trials achieved the goal of pushing the military out of politics.

Mr. Erdogan’s Islamist-based governing party and the followers of Mr. Gulen, a charismatic preacher who leads one of the most influential Islamic movements in the world and commands an empire of secular schools, had united to accomplish that task. But now that they are in open warfare, the stability of the governing party, which has been in power for more than a decade, is in question as a series of elections nears.

There have long been tensions between the two groups, but relations soured in recent weeks after the government tried to shut down private test preparation centers in Turkey, many of which are run by followers of Mr. Gulen and are important for the movement’s recruitment and finances. Mr. Gulen also commands networks of businessmen and media outlets in Turkey.

“Erdogan’s efforts to shut down the private schools was the last straw for Gulen and the Gulenists,” said Steven A. Cook, a Turkey expert at the Council on Foreign Relations. The question now, he said, is, “Are the Gulenists trying to take down Erdogan, or send a message of ‘Don’t mess with the family’? ”