The nationwide municipal elections on Sunday, the first time Turks will vote since last summer’s antigovernment demonstrations, are seen as a referendum on Mr. Erdogan’s tenure as he struggles to survive the scandal with authoritarian countermeasures, including purges of the police and the judiciary; a crackdown on the press; restrictions on access to Twitter and YouTube, where most of the damaging leaks have first appeared; and a new law that gives the government more control over the courts.

While many analysts, as well as polling data, predict that the A.K.P. will win a plurality nationwide, the percentage is most important.

Anything substantially less than 40 percent — roughly what the party won in the last local elections, in 2009 — would be considered weak. The effects could intensify dissatisfaction in the party toward Mr. Erdogan that could ultimately lead to his exit from politics. A strong showing, though, could embolden him to seek the presidency in an election later this year or, alternatively, seek to alter his party’s term-limit rules and run for a fourth term as prime minister.

Turkey is a member of NATO and a mostly Muslim country of 76 million people whose economic prosperity and, until recently, democratic progress, had been welcomed by many in the West, including President Obama, as an exemplar of stability in a region in turmoil.

Turkey also had been seen as an important strategic ally of the United States and Europe in managing the region’s many complex problems, including the civil war in Syria and Iran’s nuclear program. But with Turkey preoccupied by an internal mess, and its leaders blaming the United States and other foreign powers for destabilizing the country, Turkey’s reliability as an ally to the West is increasingly in doubt.

Mr. Erdogan, of course, will not be on any ballot Sunday, but he has campaigned as if he were, crisscrossing the country in recent weeks and holding multiple rallies, underscoring the degree to which Turkish politics — even when it comes to local municipalities — revolves around his personality.

At his rallies, if he is not blaming foreign provocateurs for his problems, he is taking swipes at the news media, putting an already intimidated press further back on its heels. But the news media is pushing back, and in an unusual move the editors of the daily newspaper Hurriyet published an open letter to Mr. Erdogan, writing, “Whatever percentage of the votes you get, it should be your and all of your duty to defuse the dangerous polarization and tensions that has spread throughout the whole country.”