"We didn't reach any agreement, unfortunately," centrist Hatenuah party chief Tzipi Livni told Israel Radio on Monday after she took part in a late-night meeting on Sunday with the heads of the centrist Yesh Atid and left-leaning Labor parties.
Livni, a former foreign minister and peace negotiator with the Palestinians, declined to discuss details of the negotiations but said she still hoped the three parties could achieve a unity pact.
Opinion polls predict that Netanyahu's right-wing Likud party, running in the election in partnership with the nationalist Yisrael Beitenu faction led by former Foreign Minister Avigdor Lieberman, will easily win the national ballot.
Livni said a joint center-left campaign would have attracted enough undecided voters, seeking an alternative to Netanyahu, to create a bloc of more than 40 seats in the 120-member parliament, topping the 37 forecast for Likud-Yisrael Beitenu.
Polls predict the three parties running separately will amass seats that number only in the mid-30s.
In the election, Israelis vote for a party's list of parliament, and no one faction has ever won a majority in the legislature.
After the ballot, Israel's president chooses a party leader to try to put together a governing coalition. That is usually, but not always, the head of the party that won the most parliamentary seats.
Netanyahu has used the prospect of a center-left union to try to win back support from traditional Likud backers who opinion polls show intend to vote for Bayit Yehudi, a far-right party led by Naftali Bennett, a former settler leader who wants to annex parts of the occupied West Bank.
"Against the left-wing bloc, you need a large Likud-Yisrael Beitenu," Netanyahu said repeatedly at a campaign event late on Sunday - a party held for hundreds of young Likud supporters at a Tel Aviv night club.
Livni has proposed that Hatenuah, Yesh Atid and Labour, should they form a bloc but lose the election, consider joining a Netanyahu-led government as an alternative to smaller religious parties and Bayit Yehudi, which has surged in recent polls.
Labour party head Shelly Yachimovich, in a separate Israel Radio interview, ruled that out.
"Whoever thinks that you can change Netanyahu from inside, meaning by sitting next to the driver's seat and pressing the brakes ... is misleading the public. As long as Netanyahu is prime minister, nothing will change," she said.
JERUSALEM (Reuters) -- Three Israeli centrist and left-leaning parties have failed in an initial attempt to form a united bloc that might have cut into Prime Minister Benjamin Netanyahu's opinion poll lead before the Jan. 22 election.