Likud members are voting in the primary elections for the party's leadership today.
Israeli Prime Minister Benjamin Netanyahu is expected to win the vote by a majority, although he faces a challenge from Moshe Feiglin, head of the ultra-right wing Likud faction, "Jewish Leadership".
Mr Feiglin has already challenged Mr Netanyahu unsuccessfully in other Likud primaries, but he won just 23.4 per cent of the vote in 2007.
Around 130,000 Israelis are eligible to vote.
Mr Netanyahu's supporters have been campaigning heavily for him, amid concerns that a low turnout could boost his rival's support.
Mr Feiglin, a hardliner, has described a future Palestinian state as "a terrorist country right in the heart of the land of the Bible".
Although he is not expected to win, a strong showing in the primary vote would be an embarrassment to mainstream Likud.
Yaron Ezrahi, a political analyst, said that if Mr Feiglin secured more than 30 per cent of the votes, it would seriously hurt Likud's image.