There is a straight fight between Goa BJP president Vinay Tendulkar and state Congress chief Shantaram Naik for the seat.
Naik, a sitting Rajya Sabha member, is seeking a third consecutive term.
As many as 18 legislators from the ruling group cast their votes after polling began in the Legislative Assembly building in Porvorim at 10 am.
During the first two hours, 36 out of the 38 legislators, who are eligible for voting, cast their votes.
The result will be announced later in the day.
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The Congress has 16 MLAs in the state.
Two MLAs - Siddharth Kuncolienkar and Vishwajit Rane - had earlier resigned.
NCP MLA Churchill Alemao has kept his cards close to his chest.
Alemao yesterday said he would not like to disclose whom he was supporting.
Tendulkar had earlier expressed confidence about his victory. "The numbers are with us," he said,
Naik also said he was hopeful of winning the election.
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