JERUSALEM, Nov 4 (Reuters) - Commentators are calling it a defining moment in Israeli politics, a stinging opening shot in a campaign to oust Defence Minister Ehud Barak as leader of the ailing Labour Party.

Sniping at Barak -- a former prime minister and an ex-general who has been widely hailed by friend and foe in Israel as a brilliant thinker -- a political rival in his own party called him an idiot on television late on Wednesday.

The only position I haven't held is punching bag, Barak said on Israel Radio on Thursday. I don't think this is the appropriate style for public discourse.

But the insult from Labour's Ofer Eini, who also serves as the head of Israel's main trade union, dominated the news and was described by political commentators as a carefully planned first strike in a battle to head Labour.

From now on, the defence minister is working on borrowed time, columnist wrote Yossi Verter wrote in the Haaretz newspaper. His hourglass is running out. Another year, more or less, and then he'll have to move aside, or be moved.

Barak's centre-left Labour, opinion polls have shown, is in bad shape, suffering from a steady decline in popularity.

Once the driving force in Israeli politics, it now holds only 13 seats in the 120-member parliament and serves as a junior partner in Prime Minister Benjamin Netanyahu's right-leaning government.

CHALLENGES

Two of Labour's cabinet ministers have already announced they would run against Barak in a still-unscheduled internal poll. Party members have accused him of failing to press Netanyahu for peace concessions to the Palestinians.

Eini latched on to a recent admission by Barak's wife that she illegally employed a Filipina as a maid to launch his attack.

You are a minister in the government, why are you bringing in a Filipina, Eini asked on Israel's Channel Two television.

You have to be an idiot to do that, he said, using an Arabic term, ahbal that commentators predicted would become a catchword for Barak's opponents in any coming election.

You can say many things about Ehud Barak -- but to call him dumb? Weizman Shiri, one of the Labour leader's political allies, said on Army Radio.

Israel's mass circulation Maariv newspaper coined Eini's assault Operation Ahbal in a front-page banner headline.

Political commentators were united in their assessment that Eini and Labour's Binyamin Ben-Eliezer, a former defence minister and Barak rival, had joined forces to unseat him.

Ben-Eliezer said on Wednesday he would favour an external candidate to take over Labour, but did not name names.

Hanan Crystal, a political commentator, said on Israel Radio the military's outgoing chief of staff, Lieutenant-General Gabi Ashkenazi, could be in line to challenge Barak if he opts to join Labour when he leaves the army early next year. (Reporting by Jeffrey Heller; Editing by Alison Williams)