A separate poll in Ma’ariv newspaper showed a similar margin – with the Zionist Union, fronted by Isaac Herzog, the Labour leader, on 25 seats and Likud on 21.
While Mrs Kastan-Keidar’s contribution to the trend is unclear, her intervention in Saturday’s rally overshadowed the main speaker, Meir Dagan, a former Mossad director, who attacked Mr Netanyahu’s unremitting emphasis on the threat from Iran, the Israeli leader’s main campaign issue, which has failed to resonate with ordinary voters.
Mrs Kastan-Keidar’s husband, Dolev, a lieutenant-colonel in the Israeli army, was killed in a fire fight after intercepting Hamas militants who infiltrated into Israel from Gaza last July, just days after persuading commanding officers to let him leave a training exercise to join the fighting.
In an interview with The Telegraph, Mrs Kastan-Keidar, a documentary film-maker and mother-of-three, described an awkward and emotional conversation with Mr Netanyahu when he telephoned her to offer condolences.
“I spoke about the need for peace,” she said. “I spoke like a little girl. I said, Bibi [Mr Netanyahu’s childhood nickname], can I ask you for something? Can you make sure there will be no more wars?
“I could hear the grief and responsibility in his voice. He did want a ceasefire with Hamas. He was reluctant to send in ground forces.
“I’m not cynical. I don’t believe a leader doesn’t care about the lives of his people and just sends them out to die. But he does nothing to prevent it. I do think he feels responsible. He just doesn’t understand.”
Mrs Kastan-Keidar, who writes a regular blog for Walla, an Israeli magazine, on coping with her bereavement, said she requested speaking time at last week’s rally because of her belief in the urgent need for a peace deal with the Palestinians.
Speaking to a nationally televised audience, she lamented an election campaign that had virtually ignored last summer’s war – which killed nearly 2,200 Palestinians and 73 on the Israeli side – and the decades-long conflict between the two sides.
“The conflict with the Palestinians has taken too many lives and the only way to stop it is to reach a peace agreement,” said Mrs Kastan-Keidar, a diminutive figure who wears her own and her husband’s wedding rings attached to a gold necklace.
Her message seemed to gain added resonance for being delivered in the same square where Yitzhak Rabin, the former prime minister who signed the now-moribund Oslo peace accords, was assassinated by a Right-wing Israeli in 1995.
Yet it failed win over Mr Netanyahu himself, who last weekend backtracked on his own previous commitment to accepting a Palestinian state – made in a speech to Tek Aviv’s Bar Ilan University in 2009. The speech was “no longer relevant” because of the danger of such a state falling into radical Islamist groups’ hands, he told journalists.
Mrs Kastan-Keidar also came under attack from a Right-wing journalist, Hagai Huberman, who accused her in a widely-read article of “killing her husband and crying that she is now a widow”.
Huberman later said his words were not meant literally but referred to her belief in the peace process, which he said led to more Israelis being killed.
But peace with the Palestinians has been raised in the campaign’s final days. Tzipi Livni, the former foreign minister and joint leader of the Zionist Union with Mr Herzog, told a rally in Beersheba this week that she would revive peace talks.
“I hope to go back to a direct dialogue with the Palestinians, so we can have direct talks and the support of the Arab countries that support us,” said Mrs Livni. “We need to get an agreement that it’s not only a piece of paper but a change in the order in the region.”