By Jeffrey Heller and Stephen Farrell
JERUSALEM (Reuters) - Israel moved closer towards holding a third election in less than a year on Wednesday, after Prime Minister Benjamin Netanyahu's main challenger failed to form a government.
Benny Gantz's announcement that he would not meet a midnight deadline following Netanyahu's own failure to put together a coalition in October deepened political deadlock at a time of mounting security and economic concerns.
There now begins a 21-day period in which Israeli lawmakers can nominate any one of the Knesset's 120 lawmakers to try and establish a coalition where others have failed.
If that fails too, an election is triggered within 90 days, raising the prospect for a weary electorate of going back to the polls after inconclusive votes in April and September.
The stalemate has shaken Israelis' confidence as conflicts with Iran and Syria deepen, and has vexed a usually friendly White House, which has had to repeatedly delay a long-awaited Middle East peace plan until an Israeli government is formed.
For Netanyahu, not securing a fifth term as prime minister also has legal implications: It may increase his vulnerability to possible indictment on corruption charges.
Israel's attorney-general is due to announce soon whether or not the conservative Likud party leader - who is Israel's longest-serving prime minister - will be formally charged following long-running police investigations.
Netanyahu denies all wrongdoing, accusing his opponents of a witch-hunt. Under Israeli law a serving prime minister does not have to step down if charged.
Gantz, a former general who heads the Blue and White party, has made much of Netanyahu's legal woes, portraying himself as a unifying centrist figure.
"In the past 28 days, I have left no stone unturned, irrespective of how small, in my attempt to form a government that would bring to the State of Israel leadership with integrity, morality and values,“ he said on Wednesday night.
“We have made great efforts towards forming a broad, liberal unity government... a government that will serve everyone - religious and secular, Jews and Arabs."
While they are largely aligned on national security, Gantz has signalled more openness than Netanyahu to a resumption of long-stalled peace talks with the Palestinians.
Netanyahu has sought to cast Gantz as a dovish novice who is not up to the task of running Israel's economy and statecraft alone.
Netanyahu issued a last-gasp appeal to Gantz to compromise, telling rightist factions that, even among its closest allies, Israel was "becoming a joke" due to its political turmoil.
"For the sake of Israel's security, for the sake of the will of the people, for the sake of reconciliation among the people, we indeed need to form a unity government."
President Reuven Rivlin had proposed a "rotation" agreement between Netanyahu and Gantz in which the Likud leader would take a leave of absence as prime minister should he be indicted.
One potential kingmaker, Avigdor Lieberman, declined to back either Netanyahu or Gantz as the deadline neared.
Lieberman, who heads the far-right Yisrael Beitenu party said on Wednesday that "both (Netanyahu and Gantz) were guilty" in failing to agree a Likud-Blue and White alliance, which he had strongly advocated.
With no unity government in sight, Lieberman said, he would deny both men the support of his party's eight legislators, effectively meaning that neither Netanyahu nor Gantz would have sufficient backing to get a working majority.
"As things stand now, we are on the way to another election," Lieberman said.
He reiterated opposition to a Netanyahu-led government that included ultra-Orthodox Jewish parties with religious influence over life in Israel, and to an administration headed by Gantz that, he said, would be dependent on support from Arab parties he described as a "fifth column".
Ahmed Tibi, a senior politician from Israel's 21 percent Arab minority, tweeted that Lieberman's rhetoric constituted "incitement" and "straight-up racism and anti-Semitism".
(Additional reporting by Dan Williams; Editing by Hugh Lawson and Philippa Fletcher)