“It can happen here.” That’s the latest meme of Turkey watchers. President Erdogan’s latest moves against his opponents raise fears that the world’s authoritarians could teach each other how to erode a rule-based state. Mr. Erdogan’s assault on Turkish institutions reminds President Trump’s foes of events at home. Critics of Prime Ministers Modi, Netanyahu, and Orban chime in: How long before America, India, Israel, Hungary, et al become Turkey? 

This morning Mr. Erdogan’s police arrested a top political rival, Mayor Ekrem İmamoğlu of Istanbul, on charges of corruption and terrorism. More than 100 of the mayor’s top aides were also picked up by paddy wagons. Yesterday Istanbul University stripped Mr. Imamoglu of his academic credentials. Having at least a bachelor’s degree is a legal requirement for Turkish presidential candidates, which all but takes Mr. Erdogan’s most viable competitor out.