Israel's premier Benjamin Netanyahu faces off against longtime rival Gideon Saar in a Likud party leadership contest Thursday that could threaten his grip on power.
A defeat for Netanyahu, 70, would be a shock, but even a relatively close result could weaken his influence over the conservative party he has dominated for 20 years, AFP reported.
Whatever the result, "Netanyahu can only lose", said Stephan Miller, a pollster who has worked on multiple Israeli campaigns.
No matter how much support Saar receives, "it will be the first time in 10 years that a group of voters on the right explicitly express their desire to get rid of Netanyahu," he said.
"If that is more than a third of the party, Netanyahu will be significantly damaged."
In power as premier for a decade, Netanyahu is facing a third general election within 12 months as well as an indictment on a series of corruption allegations.
Saar and Netanyahu have spent recent days criss-crossing the country, making their case to around 116,000 Likud members eligible to vote, though Netanyahu has not responded to Saar's call for one-on-one debates.
More than 100 voting stations were to open across the country at 9:00 am (0600 GMT) and close at 11:00 pm, with results not expected until early Friday morning.
- Corruption, election -
The winner of Thursday's vote will lead Likud into Israel's third election within 12 months.
Polls in April and September saw Netanyahu deadlocked with centrist challenger Benny Gantz, neither of them able to command a majority in Israel's proportional parliament.
Last month, Netanyahu was indicted for fraud, bribery and breach of trust in three corruption cases, allegations he strongly denies.
The primaries were called shortly after, the first internal challenge to Netanyahu since 2014.
Saar, 53, has been a senior figure in the Likud for a decade and held multiple ministries, but stepped away from politics for several years in 2014 after being politically sidelined by Netanyahu.
He is seen as slightly to the right of Netanyahu and has called for an even tougher line with the Palestinians.
A series of polls in recent weeks have indicated a Saar-led Likud might win fewer seats in a third election than under Netanyahu, but the overall rightwing bloc might be larger -- potentially enabling it to break the impasse and form a majority government.
Saar has not attacked Netanyahu personally, even hinting he would support him becoming Israel's president.
A source close to Saar insisted they were hopeful of an upset.
"More and more, the Likud rank and file are understanding the choice is between Netanyahu and being in opposition versus Saar and being in government," the source said, according to AFP.
- Facing multiple fronts -
Netanyahu has sought to paint himself as an irreplaceable leader fighting a "witch hunt" by the police, the legal establishment and the media.
He demands fierce loyalty inside Likud and has isolated and ultimately forced out multiple potential rivals.
Other party figures seen as potential future leadership candidates -- including parliamentary speaker Yuli Edelstein -- have so far chosen not to support either candidate, despite pressure.
"The way that the Netanyahu era ends is by a challenge from the right -- not the left."