Israel's ruling Likud party will hold a chairmanship primary election on Thursday to determine who will lead it in the next election.
Some 120,000 members of the party will cast their votes today, with official results expected on Friday, Xinhua news agency reported.
Prime Minister Benjamin Netanyahu is running against Gideon Saar, a Likud lawmaker and a former education minister, in one of the crucial challenges in his long political career.
Both candidates spent the day before the internal elections in energising supporters and making logistic preparations to ensure their voters would arrive at the ballots amid unusually stormy weather that hit Israel.
The two addressed their right-wing constituency.
Netanyahu announced that he will convince the White House to acknowledge Israel's sovereignty over the Jordan Valley, part of the Palestinian West Bank seized by Israel in the 1967 Middle East war.
"Only I can make it," the Prime Minister said in video footage released on his Facebook page.
Saar pledged that he will impose Israeli law on the entire settlement West Bank, a controversial move that would practically annex the disputed territory.
Local media reported that internal polls, commissioned by the candidates' headquarters, predict a landslide victory to Netanyahu.
Saar, 53, had announced earlier this month that he will run against Netanyahu, the longest-serving prime minister.
The embattled leader is facing criminal indictments in three separate corruption scandals, in which he is charged with bribery, fraud, and breach of trust. He denies any wrongdoing and says the charges are part of a "witch-hunt."
Netanyahu is currently struggling for his political survival, failing to form coalition governments after the last two national elections in Israel. However, if he wins the primary leadership polls, he is likely to stay in office at least until March 2, when the next general elections will be held.
The unprecedented parliamentary polls will be the third time Israelis would cast their ballots in less than a year, amid political deadlock that has paralysed the country's political system over the past several months.
( With inputs from ANI )