At least five other presidential loyalists also abandoned Mr. Rajapaksa to join the defection led by the health minister, Maithripala Sirisena, who also had been the ruling party’s general secretary.
Mr. Sirisena said in his resignation announcement that he had been nominated by a united group of opposition parties to be the main challenger against Mr. Rajapaksa in the Jan. 8 vote.
Others who quit the ruling alliance included the fisheries minister, Rajitha Senaratne; the education services minister, deleted Dissanayake; and M. K .D .S. Gunewardane, a deputy minister. Vasantha Senanayake and Rajiva Wijesinha, members of Parliament from the ruling Sri Lanka Freedom Party, also defected to the opposition.
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Mr. Sirisena pledged to return power to the country’s Parliament, which is now overshadowed by a powerful presidency.
“I am the common candidate,” Mr. Sirisena told a packed news briefing after resigning.
While opposition parties had pledged for months that they would field a single candidate, the success of the effort as well as the extent of defections from the ruling party were a surprise.
“This has all the potential of being a real electoral contest now,” said Paikiasothy Saravanamuttu, executive director of the Center for Policy Alternatives in Colombo. “The Rajapaksas simply cannot afford to lose power, so it promises to be a hard-fought campaign.”
Mr. Sirisena pledged to appoint Ranil Wickremesinghe, the leader of the opposition United National Party, as prime minister if he is elected president.