Members of Israel's Kadima opposition party began voting in primaries on Tuesday with pundits predicting a tight contest between current leader Tzipi Livni and her deputy Shaul Mofaz.
The polls for the center-right party's new leader opened at 10 am when Kadima's 95,000 registered members began voting at 104 polling stations around the country, officials said.
They were due to close 12 hours later at 10 pm with the final results not due out until early Wednesday.
Kadima is currently the largest party in parliament, holding 28 seats in the 120-seat Knesset, but a series of recent polls suggest the faction is likely to see that number halved in the next elections.
Kadima's current leader is a former foreign minister and one-time Mossad spy who has risen quickly through the ranks to become Israel's most powerful woman.
Her challenger Mofaz is an Iranian-born former defense minister and chief-of-staff who currently heads the powerful parliamentary committee on foreign affairs and defense.
The two last faced off during a very close leadership race in September 2008, which Livni won by several hundred votes to replace scandal-plagued premier Ehud Olmert as leader.
While most Israeli commentators were unsure who would win the race, they were unanimous that the primary vote was likely to mark the end of Kadima as Israel's biggest party, saying the outcome would see the faction split.
Kadima was founded in November 2005 by former Prime Minister Ariel Sharon, who left the right-wing Likud party to set it up in the wake of his controversial decision to pull all settlers and troops out of the Gaza Strip some three months earlier.
It has since grown to become Israel's largest party, but has taken a hammering in the polls over what commentators say has been a failure to present a concrete alternative to Prime Minister Benjamin Netanyahu and his right-wing coalition.