Life in one of Israel’s ultra-Orthodox military units does not proceed according to the usual army schedule. The morning starts with prayers just before dawn. Meals in the barracks are prepared under the strictest kosher requirements. Training is halted twice more during the day for prayers; once again for a rabbi to teach soldiers about religious texts. Unlike the rest of the Israel Defense Forces, there are no women on duty.

Many of the unit’s deeply observant members were raised to be rabbis, which is seen as the highest calling and duty. But as Daniel Rosenberg, an ultra-Orthodox who operated a heavy machine-gun, explained, sometimes a “kid doesn’t want to be a rabbi; he wants to be a fighter”.

These men, who number just a few thousand, are at the centre of a fierce debate in Israel that has driven rifts through society, peaking earlier this year when political differences over the issue shattered attempts to form a government. The unresolved dispute now hangs over Tuesday’s election.

Israel has mandatory army service but has always made an exemption for ultra-Orthodox Jews, also known as Haredi, who are allowed to continue full-time Torah study.

During the past two decades a small but growing number of Haredi have volunteered to join the military, often going against their parents’ wishes, and in many cases being rejected by their families.

“You need to have a lot of strength and power to detach from your family’s beliefs and go do something extremely different,” said Rosenberg, 21, who recently ended his time as a Haredi paratrooper. “They have no emotional help,” he said. “And then, when it comes to the field, they’re beasts … Those kids are on their own, and that gives them a lot of strength, ironically.”

The exemption policy dates back to just after the country’s founding, when 400 yeshiva students were permitted to avoid conscription.. With Haredi populations increasing to about 12% of the country’s nine million citizens, tens of thousands now avoid the military and live on government stipends.

For Avigdor Lieberman, a secular former defence minister, the issue was a deal-breaker. In May he refused to join a coalition government with ultra-Orthodox parties unless Benjamin Netanyahu agreed to force the Haredi into the army. The stalemate led to a second election being called.

It is not only in Israeli society that Haredi soldiers are controversial. The first ultra-Orthodox battalion, Netzah Yehuda, established in 1999, mainly operates in the occupied West Bank and has been embroiled in a series of abuse allegations over the years against Palestinians – including violent beatings of handcuffed detainees and even claims of electrocuting prisoners.

In March an Israeli military court convicted four soldiers from the brigade for aggravated abuse after they filmed themselves laughing and hitting two arrested Palestinians. “We’re having a party here,” one is filmed saying before slapping a blindfolded man.

In Israel, the Haredi call-up issue could paralyse the next attempt at government-building. At the very least, it has added vitriol into politics. Netanyahu’s main rival, Benny Gantz, has hinted that he might also snub powerful religious politicians if elected.

Israeli soldiers and an ultra-Orthodox Jewish man pray at the Western Wall, in Jerusalem’s Old City. Photograph: Ahmad Gharabli/AFP/Getty Images

Secular and religious Israel have always had their differences, but they are ballooning as ultra-Orthodox communities grow. Some fringe sections of Haredi society even reject Zionism, arguing there should be no Jewish state, and certainly not a secular one, before the arrival of the Messiah. In parts of Jerusalem, protests against authorities are common.

Yehuda Meshi-Zahav used to lead those demonstrations and says he was arrested more than 30 times. But his opinions changed when in 1989 he came across the aftermath of a bloody attack by a Palestinian who seized control of a public bus and forced it off a cliff near the outskirts of Jerusalem. It killed 16 people.

“That was something that shook me very deeply,” he said. “Your mind changes when you go to the scene of an attack. You understand that, in order to keep the possibility of having what we have here, everyone needs to volunteer and give what they can.”

He was one of the first Haredi rabbis to promote the idea of serving in the army, and his son, Netanel Meshi-Zahav, has just left a Haredi paratrooper unit. The 22-year-old spent a lot of his time helping so-called “lone soldiers”, men whose families had abandoned them. As his father had embraced mainstream Israeli society, he knew both sides, and also acted as an aide to non-religious officers who might not understand the Haredi background.

“Sometimes secular officers do not understand that these guys have been sitting for 18 years on a chair learning and studying the Torah,” he said. “They haven’t run 100 metres in their lives.”

With time, the soldiers caught up and, he boasts, Haredi units now often win many army sporting competitions. One secular officer he knows even became a foster carer for three lone soldiers.

Meshi-Zahav’s family supports him but he feels the pressure other Haredi soldiers feel in their communities, where they may be labelled traitors.

“When I used to leave the base, I would go through Tel Aviv. People were very kind; they would offer you something to drink. I felt really proud to have my paratrooper wings on my chest,” he said. But when he took the bus to deeply religious neighbourhoods, “the change was very clear … I could feel the anger, the tut-tut-tut.”

He understands the sentiment, but says it is frustrating: “When you are lying on the ground in the rain, they don’t understand you are protecting them.”