JERUSALEM (Reuters) - Israel's main centre-left parties may join forces against conservative Prime Minister Benjamin Netanyahu in the January 22 election that he is currently forecast to win easily, one of the challengers said on Saturday.

Israel's Prime Minister Benjamin Netanyahu pauses during the delivery of joint statements with Bulgaria's President Rosen Plevneliev (not pictured) before their meeting in Jerusalem October 23, 2012. REUTERS/Baz Ratner

Former foreign minister Tzipi Livni, head of the centrist Hatenuah party, said on Twitter that she would meet counterparts from the kindred Yesh Atid and left-leaning Labour "to discuss the creation of a 'united front' to work together to replace Netanyahu".

Opinion polls see the three parties taking around 37 of parliament's 120 seats in the vote - collectively, enough to best the some 35 projected for Netanyahu's joint rightist list and, potentially, to form the next Israeli coalition government.

Netanyahu is a two-term premier who takes credit for the relative stability of the Israeli economy and appeals to the Jewish state's burgeoning religious-nationalist sectors by championing the settlement of occupied land. He has sounded hawkish on the Palestinians and Iran but avoided big conflicts.

Israel's festering international isolation has been seized on by Livni, who as top diplomat in the former government pursued inconclusive talks on founding a Palestinian state.

The leaders of Yesh Atid and Labour, Yair Lapid and Shelly Yachimovich, are new to politics and known to much of the public from their former jobs as television commentators. Their campaigns have focused largely on social reform.

Any alliance of the Netanyahu challengers would likely require that they agree power-shares and policies in advance.

Yachimovich said this week she intended either to be the next prime minister or to sit in opposition, and that Labour would not join a Netanyahu-led government. Livni and Lapid have yet to do the same.

"A unified move by ... all those who seek to change the government will be real and meaningful only if such parties act as we did," Yachimovich said in a statement confirming that she had agreed to meet Livni.

By collectively ruling out a future coalition partnership with Netanyahu, Yachimovich said, challengers could "plant enormous hope in the heart of the public ... and bring about grassroots mobilisation for a determined and spirited struggle".

Lapid played down his scheduled meeting with Livni, telling Reuters that he would go at her invitation "because I'm a polite man". He said he had not agreed to discuss uniting the parties.

The Netanyahu government was unfazed by Livni's initiative.

"I wish that the other side, to the left, would coalesce, because that would hone the differences between us," Vice Prime Minister Moshe Yaalon of the ruling Likud said in a speech.

In an apparent dig at Lapid and Yachimovich, Yaalon rued "the immodesty and immaturity in the desire of certain people to jump straight into the cold water of being prime minister, without passing through any stations along the way".

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