Erdogan survived the Gezi Park protests earlier this year with relative ease, but this may be a more dangerous threat. Gulen lives in exile in the U.S., but has a large network of followers in Turkey, controls the country's biggest newspaper, and has deep roots in the courts and law-enforcement agencies.
Earlier today prosecutors detained people close to the government in connection with corruption and gold-smuggling investigations. Among those held are the sons of at least two of Erdogan's cabinet ministers; a construction mogul; and the chief executive of the state-controlled Halk Bank, whose share price fell 12 percent on the news.
This is one occasion when it makes sense to believe the conspiracy theories. The political threat to Erdogan is clear and judging by their comments on Twitter, secular Turks are delighted that religious conservatives are tearing at each other.
Gulen and his supporters were part of the coalition that made Erdogan and his ruling party, Justice and Development, the dominant force in Turkish politics. Together, they used the courts to crush the so-called deep state -- the military officers, security officials, journalists and academics -- that ruled Turkey behind the scenes before Erdogan came to power.
With that unifying enemy gone, though, what remains are two competing power centers at a time when Erdogan is trying to consolidate his electoral base ahead of presidential elections next year. Gulen, for example, publicly opposed Erdogan's handling of the Gezi Park protests, and recently referred to the country's leader as a "the Pharaoh" and "Croesus."
Erdogan lost interest in the military cases and now opposes them, after establishing his control over the armed forces. He more recently began to attack the Gulen movement, threatening closure of its ubiquitous schools that provide the movement with revenue and followers. Even if the corruption cases are strong, it seems clear -- and is assumed as self-evident in Turkey's news media -- that the arrests were a direct response to Erdogan's move against the Gulen movement. The investigations are being led by the same prosecutors who pursued largely fabricated cases to cut the military down to size.
This is a peculiarly Turkish showdown between Gulen's nationalist religious movement, which preaches tolerance and making peace with modernity, and Erdogan, a former Islamist. The two are competing to redefine the sometimes aggressively secularist Turkey that Mustafa Kemal Ataturk built, to be more like the Ottoman state Ataturk rejected. Neither, unfortunately, appears to have much respect for the rule of law.
Turkey might well benefit if the Justice and Development Party were to split in an orderly way, creating a more balanced political system. There isn't much reason to welcome an escalation in this fight, though. Unless the two sides find a way to settle, this may prove disruptive for the country's economy.
Government tax inspectors were already attacking secular business empires, such as that of the Koc family, for their alleged role in helping the Gezi Park protests in June. Now prosecutors are pursuing companies close to the government for alleged corruption. Expect the government to find a way to go after Gulen's business supporters, too.
Erdogan remains popular, powerful and a formidable political tactician. Whatever happens, though, this conflict demonstrates the weakness of the politicized law-enforcement institutions that he and his party inherited, and chose to continue misusing for their own ends, instead of reforming them. Erdogan is now paying a price for that choice.
(Marc Champion is a Bloomberg View editorial board member. Follow him on Twitter.)