[caption id="attachment_155997" align="alignright" width="205"] Israel's Prime Minister Benjamin Netanyahu casts his ballot for the parliamentary election at a polling station in Jerusalem March 17, 2015.[/caption]

(Reuters) - Prime Minister Benjamin Netanyahu's march towards becoming the longest-serving leader of Israel could be halted on Tuesday in an election that has exposed public fatigue with his stress on national security rather than socio-economic problems.

Surging rhetoric against Iran and the Palestinians has done little to close Netanyahu's lag behind center-left opponent Isaac Herzog in opinion polls. Should Herzog narrowly win the ballot as predicted, he would be the likely first pick to form the next government.

That would not rule out the coalition-building task reverting to Netanyahu, if Herzog fails to win enough support in a rightist-dominated parliament.

Much will depend on which candidate the smaller, centrist parties choose to crown, and the leaning of a joint list uniting Israel's four Arab parties, which is expected to come in third.

Dubbed "King Bibi" by Time magazine just three years ago, Netanyahu, 65, has cast the threat to his reign as a foreign-orchestrated bid to install an Israeli leader who might yield to Palestinian statehood or nuclear diplomacy with Iran.

Netanyahu, his wife and one of his sons joined the first wave of voters, casting his ballot in Jerusalem. He acknowledged in brief remarks to reporters that his opponents' lead had widened.

"To prevent the rise to power of a left-wing party, there is only one thing to do - close the gap," he said, hoping to rally religious-nationalist voters to his troubled Likud party.

Herzog, the head of Israel's Labour party, and his running mate, ex-peace negotiator Tzipi Livni, have accused Netanyahu of using security scares to distract from social issues like the high cost of living emphasized in domestic debates.

"Netanyahu is in a great panic, whereas for Tzipi Livni and me what is foremost is the good of the country," Herzog, 54, said late on Monday. The election, he said "is a choice between change and hope, and disappointment and downfall."

NO TO PALESTINE

In what appeared to be a last-ditch pitch for far-right votes, Netanyahu on Monday said no Palestinian state would arise under his watch if he won a fourth term in top office. He has so far served nine years, second in duration only to Israel's founding father, David Ben-Gurion, who was premier for 12 years.

The United States, which oversaw now-stalled talks on a two-state peace deal between Netanyahu and Palestinian President Mahmoud Abbas, reserved comment on the Israeli leader's volte-face, citing reluctance to weigh in so close to the vote.

"We will work with the winner of the election," said U.S. State Department spokeswoman Jen Psaki, echoing similar remarks by Abbas last week.

The Obama administration's ties with Netanyahu were already strained by his opposition to nuclear negotiations with Iran, tensions that crested in his March 3 speech to the U.S. Congress that many of the president's fellow Democrats sat out in anger.

As on the Palestinian issue, Netanyahu said he was motivated by the risk to Israel's survival. However, one poll found that most Israeli voters were unmoved while a minority said they were less likely to back him over his open defiance of Washington.

Mild-mannered and untested in statecraft, Herzog favors re-engagement with the Democratic White House and the Palestinians. But he has steered clear of promising peace or a satisfactory resolution to the stand-off over Iran's nuclear program.

Asked whether Herzog has the "killer instinct" that many Israelis expect in their prime ministers, the candidate's older brother Michael, a retired general and Defense Ministry chief of staff, was circumspect.

"There are different kinds of leadership," he told Army Radio.

In his own election-eve gambit, Herzog announced the cancellation on Monday of a rotation agreement with Livni whereby they would each serve as prime minister if victorious. The deal had turned off some potential voters, Herzog explained.

Polling stations close at 2000 GMT (1600 ET), when Israeli media are expected to publish exit polls. Initial results will be published at 2100 GMT (1700 ET).

By Reuters