Israel's Prime Minister Benjamin Netanyahu (R) and Isaac Herzog, Co-leader of the centre-left Zionist Union, are pictured together as campaign billboards rotate in Tel Aviv, March 9, 2015. (Photo: Reuters)
Less than a week before Israel's second general election in two years, Isaac Herzog's centre-left Zionist Union opened up a lead on Prime Minister Benjamin Netanyahu's rightwing Likud, polls showed Wednesday.
The March 17 vote comes as Israel faces major domestic and international challenges, all of which will have to be addressed by the next government - among them the Iranian nuclear threat, the deadlock in peace efforts and a looming Palestinian legal move at the International Criminal Court.
Experts say the vote will largely be a referendum on the six-year tenure of Netanyahu, who has made security the centrepiece of his campaign.
The Israeli leader last week gave a controversial address to the US Congress on the threat that would be posed by a nuclear Iran, in a move he hoped would boost his support ahead of the vote.
But a series of polls published this week show an erosion in support for Netanyahu's ruling Likud party, which for weeks had been neck-and-neck with its centre-left rival, with both hovering at around 23 or 24 seats.
According to a survey by Israel's army radio, Herzog's list was seen taking 24 seats to Likud's 21, indicating an erosion in support for Netanyahu's faction.
The poll found that the centre-left and potential allies would take 54 mandates compared with 58 for the rightwing and religious parties within the 120-seat Knesset, or parliament.
The centre-right Kulanu faction, which has not said whether it would back Netanyahu or Herzog and could play the role of kingmaker, is seen taking eight seats.
A similar poll by Channel 2 television released late on Tuesday gave the Zionist Union 25 seats to 21 for Likud, with the centre-left and allies taking 55 seats to 57 for the right. It too forecast eight seats for Kulanu.
Potential allies for the Zionist Union include the Joint Arab List, the centrist Yesh Atid and the leftwing Meretz.
But even if the lead does translate into concrete votes, Herzog is far from guaranteed to head the next government.
Under Israel's electoral system, the task of forming the next government does not automatically fall to the leader of the largest party or list -- rather to the politician who has the best chance of putting together a coalition government.
Proportional representation means the smaller parties are often well placed to play kingmaker, tipping the balance one way or another.
And in this respect, Netanyahu has a clear advantage over Herzog as he will find it much easier to piece together a majority.
Among his natural allies are the far-right Jewish Home, the hardline Yisrael Beitenu, ultra-Orthodox parties Shas and United Torah Judaism, and the far-right religious Yahad faction.
Asked by public radio whether he would back Netanyahu or Herzog for premier, Kulanu's Moshe Kahlon, a former Likud minister, refused to be drawn.
With the pressure on, Netanyahu sought to rally voters in a posting on his Facebook page.
"This time you must vote Likud - because if the gap between Likud and Labour isn't closed, there is a real danger that Tzipi and Buji (Herzog) will head the next government with the support of the Arab parties," he said.
The Zionist Union is list which combines Herzog's Labour party with the centrist HaTnuah headed by former justice minister Tzipi Livni.
Netanyahu also warned against wasting ballots by voting for smaller rightwing factions not committed to work with Likud, such as Foreign Minister Avigdor Lieberman's Yisrael Beitenu and Kulanu.
Speaking to army radio, Herzog insisted he was the only leader "able to replace Netanyahu" although he dodged a question about whether he would agree to form a unity government with Netanyahu.
"Wherever I go, they tell me not to go with Bibi (Netanyahu), with the ultra-Orthodox or with the Arab parties," he said. "But we have to keep in mind Israel's political map."
Israel's last serving Labour prime minister was Ehud Barak (1999-2001) who took office after defeating Netanyahu following the latter's first term as premier.