He is facing long-term rival Gideon Saar in internal Likud party elections.
Although Netanyahu is expected to win, it comes at a time of mounting difficulties for the veteran leader.
He faces trial on bribery and corruption charges, as well as a third national election within a year after two inconclusive results.
Netanyahu has campaigned tirelessly, even though he is widely expected to triumph, because he wants to win big – otherwise it will look like his grip on the party is weakening.
The prime minister still has solid support in Likud, but this is the most serious internal challenge he has faced in a decade of power and shows cracks in the party’s united front.
There was a moment of drama at a campaign rally in the southern city of Ashkelon on Wednesday night when Netanyahu was ushered off stage by bodyguards after an air raid siren…