Members of Israel’s ruling party, Kadima, go to the polls today to choose a new leader, with Foreign Minister Tzipi Livni the front-runner and Transportation Minister Shaul Mofaz her nearest rival.

The winner is expected to move quickly to try to form a coalition government, as having a sitting prime minister will improve the centrist party’s chances of winning a general election. Recent polls have heavily favored the right-of-center Likud Party, headed by Benjamin Netanyahu, in such an election.

Today’s primary race is unprecedented in the history of Israel’s complex parliamentary system. Prime Minister Olmert, who has been the target of several police investigations, announced this summer that he would step down after Kadima chooses a new head. But if the winner fails to put together a coalition quickly, Mr. Olmert could stay on as prime minister for several months.

“The most important task for whoever wins the Kadima primary — be it Livni or Mofaz — is to quickly form a coalition government,” Israel Radio’s top political analyst, Hanan Crystal, told The New York Sun. “But even if he or she can do that, they would need to secure the coalition, which could collapse either by failing to pass the state budget next March or by losing in a no-confidence vote” of 61 legislators in the 120-member Knesset.

Kadima has 73,000 members who are eligible to vote in the first-ever primary for the party, which was founded in late 2005 by Prime Minister Ariel Sharon, along with former members of Likud and the center-left Labor Party. In addition to Ms. Livni and Mr. Mofaz, two other Kadima members, Public Security Minister Avi Dichter and Interior Minister Meir Sheetrit, are vying for the party’s leadership.

Pollsters have been predicting a victory for Ms. Livni, who is said to have the support of as many as 47% of party members. But the more hawkish Mr. Mofaz, who has strong support among party-affiliated unionists and small-town party activists, has been promising that he will win with 43.7% of the vote. Messrs. Dichter and Sheetrit lag in the polls, but their presence on the ballot could force a second round of voting next week, if neither of the front-runners secures the support of more than 40% of all votes cast.

In such an unprecedented race, a “high turnout favors Livni,” Mr. Crystal said. “If more than 40% of eligible voters cast ballots, I would be very surprised if she loses.”

The complicated political situation after today’s primary — and the need to put together a coalition government swiftly — has already forced Ms. Livni to cancel a planned trip to New York next week to participate in the annual U.N. General Assembly session.

Mr. Olmert also has decided to skip the event, and Israel could be represented by President Peres. But under Israeli law, one of the president’s duties is to ask candidates to form a new government — which means that Mr. Peres may be forced to cancel his trip at the last moment, as well.