With the latest polls suggesting that momentum is shifting to Isaac Herzog, it appears that Israelis are beginning to imagine what a month ago was not imaginable: Life without Bibi.

But a fifth of voters say they are undecided, and the peculiarities of Israel’s electoral system suggest there’s little use in trying to call Tuesday’s elections. There are, however, some things worth thinking through as we ponder possible outcomes.

* Vote totals don’t trump all. In Israel’s political system, what matters most is who has the capacity to form a coalition. The party leader who gets the most votes (or seats in the Knesset) does not necessarily become the next prime minister. Such was the fate of former Kadima head Tzipi Livni, who outpolled Benjamin Netanyahu in 2009 but was unable to assemble a workable coalition. The math that emerges from all parties competing for Knesset seats would seem to favor the rightist Likud Party because there are more natural ideological partners, though the centrist parties that will constitute critical swing votes are doing well. For Labor Party leader “Bougie” Herzog to escape Ms. Livni’s fate, he needs to open a sizable spread, probably more than the three-point lead polls show he has over Mr. Netanyahu’s Likud. Five points would give him the margin and the momentum to have the first, and perhaps most compelling, shot to assemble a new government.

* Another March surprise is unlikely. Mr. Netanyahu’s victories in 1996 and 2009 came late in the campaign, largely as a result of security issues related to Hamas terror activities and war in Gaza. Some might argue that Mr. Netanyahu’s speech to Congress was this year’s March surprise. But that gave him only a slight bump in the polls, which has already disappeared. This close to Election Day, it’s unlikely a crisis will emerge before votes are cast. (These days, of course, one never knows.) In a closely fought election, some dust-up with Hezbollah or Hamas could turn the tide. Mr. Netanyahu’s security credentials were somewhat tarnished by the inconclusive end to last summer’s Gaza war. But his opponent has little street cred on security issues in a political culture that values it.

* All about Bibi. After almost nine years with Mr. Netanyahu, many Israelis seem ready for a change at the top. Sixty percent of respondents to a Jerusalem Post poll in December said that they didn’t want Mr. Netanyahu to continue as prime minister. At least 43% of Israelis polled after his speech to Congress felt that it wouldn’t affect their vote. For better or for worse, this election turns on how Israelis feel about the prime minister. Since firing his foreign and finance ministers last year, Mr. Netanyahu has sucked up all the political oxygen, whether on matters as weighty as Iran and a Palestinian state or issues as absurd as the cost of takeout food or redecorating his residence. For a rising politician, this would be heaven. But Bibi is playing on the back nine. The good news for him is that, though the gap is narrowing, more Israelis still consider him a suitable prime minister than the youthful and untested Mr. Herzog. The bad news is that Bibi is a well-worn, known commodity.

* Herzog as the default candidate: Labor hasn’t won an Israeli election in 16 years–but a Labor candidate has consistently polled over the 20-seat baseline necessary to have a chance of forming the coalition government. Labor is the storied political party that founded the state of Israel and helped lead it through tumultuous times before insularity, elitism, and tired messaging cost it its edge; new voters, including Jews of Middle Eastern origin who wanted a share of the pie, did not feel included. Mr. Herzog has not run a strong campaign; he lacks the fire and fight of a charismatic politician. Still, he has gained some traction thanks to being identified with the social and economic issues that many Israelis care about more than an Iran nuclear deal. Mr. Netanyahu is vulnerable, but it’s not clear whether dissatisfaction and fatigue with Bibi create enough space for Mr. Herzog and centrist swing parties to assemble a sustainable government. Put another way: Are Israelis willing to risk a change in the hopes of a getting a leader who can bring greater prosperity, security and better relations with the world–and still protect their interests in a turbulent Middle East?

*Will a new government be able to make decisions? At least three outcomes are possible after Tuesday: a rightist Netanyahu government with a narrow majority; a narrow, center-left Herzog coalition; or some kind of unity government. It’s not clear whether any would be sustainable or able to make decisions on the issue that the Obama administration and the rest of the world want to focus on: What do about the Palestinians? The first government has no incentive; the second may have a kinder, gentler face–but could lack the capacity. And when it comes to decisions on sensitive national security issues, Israeli unity governments don’t have much of a track record.

No matter who gets the most seats on March 17, a prolonged struggle is coming to put together Israel’s next government.

The opinions expressed here are solely those of the author.

This article was originally published in The Wall Street Journal’s Washington Wire.