BEIT-UL-MOQADDAS (AP) -- Labor Party members cast ballots Monday for their leader, a contest that could have serious implications for the makeup of Prime Minister Ehud Olmert's governing coalition and his own political survival.

The two front-runners in the race — former Prime Minister Ehud Barak and political newcomer Ami Ayalon, a dovish former internal security agency head — have said they would work to oust the embattled prime minister, who has been discredited by his handling of last summer's war in Lebanon.

Ayalon has pledged to lead the centrist Labor out of its year-old partnership with Olmert if the prime minister's Kadima Party doesn't choose a new leader. Should the 19-member Labor faction bolt, the prime minister could be forced to plug the hole with religious or hawkish parties that take a hard line toward the Palestinians. It could also renew pressure on him to resign.

About 104,000 people are registered to vote Monday. Polls close at 9 p.m. local time and final results are expected about five hours later.

The latest opinion polls have shown Barak and Ayalon in a tight race to succeed the current party leader, Defense Minister Amir Peretz, who places a distant third. Two other candidates lag far behind.

Polls suggest that no candidate will receive the 40 percent needed to win in a single ballot. That would set up a June 13 runoff between the top two vote-getters that polls say Ayalon would win.

Despite their criticism of the prime minister, both Ayalon and Barak appear reluctant to push too hard for his immediate ouster. Opinion polls show that both men would lose an election to former Prime Minister Benjamin Netanyahu, head of the hard-line Likud Party.

Labor's final decision on whether to remain in the coalition rests with the party's central committee. Many committee members are reluctant to leave the government, either because they want Labor to hold on to its Cabinet seats or because they fear Netanyahu would be re-elected.

As a political newcomer, Ayalon could use some time to build his popularity inside Labor and with the general public.

Barak, unseated as premier after less than two years following the eruption of the ongoing Palestinian uprising seven years ago, needs to rehabilitate his tarnished public image.

Barak, a former military chief and the country's most-decorated soldier, and Ayalon, who also served as a former naval chief, are both expected to seek the defense portfolio.

Peretz, deeply unpopular after last year's war in Lebanon, has already said he wants to leave the defense post after the primary.

A recent government probe of the war singled out Olmert for especially harsh censure. He managed to quash an incipient rebellion against him in Kadima, but could find himself on the political skids again in August, when the war probe submits its final report.