Israel's Knesset parliament opposition party on Tuesday holds primary elections to pick a challenger to Prime Minister Benjamin Netanyahu in the upcoming general elections.

Kadima (Forward), currently the biggest Knesset party, is holding 28 out of 120 seats in the unicameral legislature - a hairsbreadth ahead of Netanyahu's Likud with 27.

However, Netanyahu's governing coalition is larger than that cobbled together by Kadima party chief, Tzipi Livni, relegating the latter to the opposition bench since the 2008 elections.

The outcome of the primary will be determined between two bitter rivals: Livni, who is a former foreign minister, and former army chief of staff and defense minister, Shaul Mofaz.

Some 95,000 party members are expected to cast their ballots in what is turning into a noisy, neck-and-neck race between the two.

Former prime minister Ariel Sharon established Kadima in 2005 as a centrist party, after his unilateral disengagement plan from Gaza shook Israeli politics and society to the core, and brought about his bolting Likud - a party he himself set up with late leader Menachem Begin, in the early 1970s.

While Kadima is still Israel's largest party, acrimonious inner disputes emerged after Livni's decision to stay in the opposition, which together with acrid power struggles have brought Kadima to face its biggest crisis yet.

Externally, recent unflattering polls indicate a damning 50 percent drop in the party's political support among Israeli voters. What's more, a bevy of new and attractive centrist politicians are eyeing Kadima's own sectoral demographic: largely middle-class, secular, urban liberals. That prospect places Kadima's political existence in the ballot box bullseye.

Mofaz' supporters blame Livni for the electoral crash-and-burn.

"It's not only her fault, but as the party's chairman she has a bigger responsibility," Kadima lawmaker Yohanan Plesner told Xinhua on Tuesday.

"The deterioration in the polls is expressing criticism over our failure to pose a coherent alternative to Netanyahu's ultra religious and ultra right wing coalition," Plesner explained.

Formerly one of Livni's inner circle in the beginning of their term, Plesner broke ranks, and threw his support behind Mofaz.

"I think that he has the security experience that is crucial for Israel these days, and he also has the social sensitivities and the abilities to attract voters from the right wing," Plesner added.

On the other hand, Livni's supporters are waving their own poll figures claiming that her leadership will net them more Knesset seats than Mofaz.

"She has the bigger chance of assembling a significant amount of electorate, and to challenge Netanyahu in the future," Kadima's Nachman Shai said.

He noted that in Israeli politics, the prime minister is the one who must gain the support of other parties in order to form a coalition.

"Livni can unite with parties from the left and form a bigger block," Shai said, believing that the party's appeal will attract precisely the mirror image of the electoral map.

"Once the elections are determined, the party will look and act differently, and we will regain the (lost) support," Shai said.

The political daggers unsheathed between the two contenders are sharp and personal, placing future cooperation in major doubt. Most political analysts here predict that the loser, to be announced Wednesday, will resign the party.

It is unclear yet whether Kadima's primaries will provide the jolt of political caffeine the moribund party needs to reclaim its parliamentary prowess.

Nevertheless, Wednesday winner will be the "last man - or woman - standing" with the political muscle to threaten Netanyahu's leadership.