Who will win in Israel Tuesday? Meet the unlikely kingmaker.

With Israelis heading to the polls for their second parliamentary election in a half year, an ultranationalist firebrand has emerged as a potential kingmaker who will decide whether Benjamin Netanyahu gets another term as prime minister or is replaced by a centrist challenger.

Avigdor Lieberman, an immigrant from the former Soviet Union and a former defense minister who heads the Yisrael Beitenu party, gained notoriety over the last decade as a hard-liner regarding Israel’s Arab minority and Palestinians in the West Bank and Gaza Strip.

But in elections last spring, Mr. Lieberman refused to join a right-wing coalition led by his former partner and old boss Mr. Netanyahu, citing a falling out over the influence of religious parties.

And in the campaign leading up to the vote Tuesday, the nationalist stalwart has refashioned himself as a champion of Israelis disturbed by the efforts of ultra-Orthodox, or Haredi, politicians to promote religiously inspired policies.

In doing so, Mr. Lieberman has plunged straight into the country’s fractious divide over religion and state while turning on Mr. Netanyahu. The 61-year-old Moldovan-born politico accuses the prime minister of conspiring with the religious parties by agreeing to their demands to exempt yeshiva seminary students from Israel’s mandatory military draft and to promote policies that jibe with halacha, Jewish religious law.

Yisrael Beitenu campaign ads feature images of Mr. Netanyahu surrounded by politicians from Israel’s Orthodox parties who have become lightning rods for secular Israelis.

“On Sept. 17, the Haredim will all enlist. But they won’t be enlisting in the Israeli army,’’ one social media video warns with images of hordes of ultra-Orthodox men dressed in their traditional dark suits and white shirts. “If you don’t vote, don’t be surprised if a halacha state is established. Only Mr. Lieberman can stop a halacha state,’’ the video declares over an ominous soundtrack.

“He has become the leading voice against the ultra-Orthodox,” says Stephen Miller, an independent pollster. “He did that loudly when he was the reason that Netanyahu couldn’t form a coalition, and he has continued to sing that note” during the campaign. “This is a powerful voice in Israeli politics, and Lieberman has capitalized on it.”

The prime minister is especially vulnerable because Israeli Attorney General Avichai Mandelblit plans to indict him on corruption charges, subject to a hearing that’s set for two weeks after the election.

Moreover, there’s the challenge from former army chief Benny Gantz, whose Blue and White party is running neck and neck with the prime minister’s Likud party. Mr. Lieberman has signaled he could join a coalition led by Mr. Gantz.

The turning point

Earlier in the year, Mr. Lieberman’s party, whose main constituency is Russian-speaking Israelis, appeared in danger of falling short of the minimum percentage to win seats in the parliament, the Knesset. After the April 9 election, he initially backed Mr. Netanyahu’s attempts to form what was supposed to be a new government coalition of religious and right-wing parties.

But in the course of the coalition horse trading, he demurred and accused Mr. Netanyahu of caving to the demands of the ultra-Orthodox. The move denied Mr. Netanyahu the 61-seat majority necessary to form a government.

Outraged, Mr. Netanyahu accused Mr. Lieberman of being a “leftist.” And rather than allow another lawmaker a chance to form a government, Mr. Netanyahu and other lawmakers decided to dissolve parliament and call a second election.

A Sept. 9 poll, by Israel’s Kan public television broadcaster, showed Mr. Netanyahu’s bloc of parties with 58 seats, while center-left and Arab parties got 53. That leaves Mr. Lieberman – who polls at nine seats and has been pushing for a unity coalition with the two largest parties (without the ultra-Orthodox parties) – as the potential tipping point between the sides.

Draft exemptions for seminary students are not the only point of contention. Ultra-Orthodox rabbinic councils in Israel dictate policy over marriage, divorce, Jewish conversion, and dietary laws. That engenders bitterness with secular Israelis, among them hundreds of thousands of Russian speakers whom the rabbinic establishment doesn’t recognize as Jewish. And ultra-Orthodox families are large, making them the fastest-growing demographic group in the country.

“Lieberman has forced us as a country to look differently at how its governed. ... The religious are taking over the policies of the government,’’ says Joshua Shuman, a Virginia-born public relations executive who wears a yarmulke but believes Israel should have more of a separation of religion and state.

Mr. Shuman says he has always voted for left-wing or centrist parties. He believes, however, that Mr. Lieberman acquitted himself responsibly during tenures as defense and foreign minister.

“I don’t vote for the right, but Lieberman doesn’t seem too right any more, he seems like he’s moved to the center.”

“I don’t understand his agenda”

Many Israelis, however, see Mr. Lieberman as a political opportunist who above all wishes to ascend to the premiership. He dropped a broad hint at that desire in July.

And critics say Mr. Lieberman hasn’t delivered on many of his promises, such as his vow to take a more aggressive approach to Hamas, the militant group that rules the Gaza Strip, and to put an end to cross-border attacks.

“I don’t understand his agenda. He’s changing all the time,’’ declares Odelia Ben Sasson, a resident of southern Israel who says her children have been traumatized by repeated rocket sirens. “He never did anything about the situation in Gaza when he was minister of defense. He talked a lot.”

Mr. Lieberman was the director general of the prime minister’s office when Mr. Netanyahu first gained the premiership in the 1990s. He broke away from Likud to establish Yisrael Beitenu (“Israel is Our Home”) to tap into a constituency of 1 million Russian-speaking immigrants. But after the 2013 election when Likud and Yisrael Beitenu ran jointly and underperformed, they went their separate ways.

Mr. Lieberman is known for embracing provocative positions outside the Israeli consensus. A decade ago, he pledged to strip the citizenship of Arab citizens deemed disloyal. Later he threatened to assassinate a top Hamas leader and bomb Egypt. He is also something of a maverick on the right because he supports parting with the West Bank, although he also advocates redrawing the border to cede Israeli Arab towns to a Palestinian entity and annexing settlements to Israel.

By refocusing his campaign on issues of religion and state, Mr. Lieberman is appealing to Israelis in the political center and left – in addition to the secular right wing. Recognizing that appeal, Mr. Gantz’s party has mimicked Mr. Lieberman in the final weeks of the campaign, with giant billboards promising a “secular national unity government.”

Surprised supporters

“If you would have asked me six months ago if I might vote for Lieberman, I would have said that you’re crazy. In this election, he speaks to the issues that I think matter,’’ says Noam Rapaport, a 49-year old marketing manager, who votes for centrist parties and was won over by Mr. Lieberman’s decision to walk out on coalition talks last May.

“I don’t think the right or left in Israel is so different from each other. Seventy percent of the population share the same ideas. The main issue is the difference in the amount of money that the Haredim get.’’

On the secular right, Mr. Lieberman has tapped into a longtime undercurrent in Israeli politics that goes back to the liberal nationalists who founded Likud.

“Lieberman found the magic formula: to be right wing and anti-Haredi. Muslim Arabs and the ultra-Orthodox are the two most disliked groups among Israeli Jews,’’ says Shimon Rosner, the author of a book exploring Judaism in Israel.

“Lieberman is the first to realize that you can use unfriendly terms when you speak about Arabs and use the same term for the ultra-Orthodox. He’s placed himself as the only person who is suspicious of both groups.”