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State Senator John J. Flanagan, who until this year led the Republican majority of the state’s upper legislative chamber, will miss the beginning of this year’s session in Albany in order to complete an alcohol rehabilitation program, he announced on Friday.

Mr. Flanagan, who assumed the leadership of the chamber in 2015, had first sought treatment for alcohol dependency in 2017. In a statement on Friday, he said that he had recently recognized he needed to seek additional help and would miss the start of session, which begins on Wednesday, as a result of a “thorough rehabilitation and recovery program.”

“No man or woman is perfect, but it does not mean we all shouldn’t strive for continuous and daily improvement. I will attempt to do that with every fiber in my body,” Mr. Flanagan, who represents central Long Island, said in the statement. “This brief period of time away is necessary for my overall well-being, but will in no way impact my ability to serve my conference or my constituents.”

The Republicans decisively lost their slim majority in the Senate in November’s election, and with it their last toehold of power in state government. They will now enter the session in their weakest position in a decade, and without their leader. Senator Joseph A. Griffo, who represents Lewis and Oneida Counties upstate, will oversee the Republican conference until Mr. Flanagan, 57, returns.