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Prime Minister's Grip on Turkey Is Tested

Corruption Probe, Approaching Election Expose Rifts in Erdogan's Fractious Coalition

ISTANBUL—The broad coalition of former underdogs that swept Prime Minister Recep Tayyip Erdogan to victory in three successive elections is cracking, threatening to undermine not only his party but also the economic progress built on a decade of political stability in Turkey.

The internal power struggle burst into the open Dec. 17 when prosecutors unveiled a major investigation into alleged bribery and other charges, targeting dozens of people close to Mr. Erdogan and his allies.