For the first time in Israel’s history, all 15 of its Supreme Court justices will crowd onto the bench on Sept. 12 to hear a case together. The reason: This one is so momentous that it could not only decide the powers of the court itself but also kindle a constitutional crisis.

The 15-member court — which meets in a graceful building of beige stone, straight lines and arches on a hill in Jerusalem alongside Parliament — includes secular liberals, religiously observant Jews and conservative residents of Jewish settlements in the occupied West Bank. One justice is an Arab Israeli; six are women, including the court’s president.

The justices will be scrutinized like never before as they begin hearing an appeal against the first part of a judicial overhaul that the government pushed through Parliament in July, angering many Israelis and stoking street protests across the country.

Many Israelis fear that the overhaul will weaken the court as a check on the government, currently the most right-wing and religiously conservative in Israeli history; accelerate a rightward shift of the judiciary that started almost a decade ago; and make it more politicized and less independent.

The government has primed itself for battle against the court by portraying it as a bastion of a secular, left-leaning elite and a closed club out of touch with changes sweeping the country. Experts say that characterization has not been true for years.

Ayelet Shaked, a former justice minister and right-wing politician, said the court was “very liberal and progressive” when she took office in 2015. But, she said in an interview, “I made it my goal to diversify the Supreme Court and make it more conservative, and that’s what I did.”

During her four-year term as justice minister, Ms. Shaked led the judicial selection committee and used her sway to reach deals with other members and bring in candidates of her choice. Now, she said of the court, “It is more balanced than before.”

Michael Sfard, a human rights lawyer and political activist who has represented Israelis and Palestinians in the Supreme Court, agrees that the balance has shifted. The court is “much more right-wing, pro-settler and nationalist today than it was 20 years ago,” he said.

With its judicial overhaul, the hard-line coalition led by Prime Minister Benjamin Netanyahu wants to shift that balance further to the right by having more control over the choice of the justices sitting on the Supreme Court and ultimately to grant less power to the judiciary and more to the elected government.

The hearing in September, during which the court will essentially be deciding whether to accept a curbing of its powers, heralds a potential showdown between the top judicial authority and the ruling coalition, and could fundamentally reshape Israeli democracy.

The court’s president, Chief Justice Esther Hayut, prompted an uproar in January when she excoriated the government’s judicial overhaul plan as an “unbridled attack on the judicial system” that would “deal a fatal blow” to its independence.

However, it is unclear how the court will rule because the judges are officially tasked with upholding the law and generally avoid making public statements on political issues.

Israeli human rights lawyers say that the court’s rulings have become increasingly conservative. They point to decisions such as one from 2021 allowing colleges to offer gender-segregated courses to accommodate ultra-Orthodox students, in which the judges rejected the argument that the practice impinged on principles of equality.

Still, human rights groups say the court is an important backstop; this month, for example, the court indicated that it would intervene to ensure that an existing adoption law was not used to discriminate against same-sex couples.

“The court is still the only platform that defends human rights in Israel,” said Noa Sattath, the executive director of the Association for Civil Rights in Israel, one of the groups that have petitioned the court to strike down the government’s judicial legislation.

The court has become more transparent in the way it works, including livestreaming important hearings, said Tzipi Livni, who has twice served as justice minister. Nor does it operate like an old chums’ network, she said in an interview, despite the claims of some members of the coalition.

“The government is complaining about things that existed decades ago and now don’t,” she added.

Often, the court walks a fine line between the demands of different segments of Israeli society and respect for government policy. It has long avoided explicitly ruling on the legality of Jewish settlement in the occupied territories, which Palestinians and most of the world consider a violation of international law. But the court has often slowed settlement growth by trying to protect the property rights of individual Palestinian landowners, irritating critics on the right.

Staunch liberals on the bench, such as Justice Uzi Vogelman, deputy president of the court, particularly annoyed a previous Netanyahu-led government by quashing a law in 2014 that would have allowed for lengthy incarceration of African migrants who had entered Israel illegally and ordering the release of all the detainees being held in a desert detention center.

Four out of six Supreme Court slots that opened up during the tenure of Ms. Shaked, the former justice minister, were filled by candidates she identified as conservatives, including Yael Willner, the first female justice to cover her hair in line with religious modesty rules. A fifth with conservative leanings was selected under a subsequent government in which Ms. Shaked served as interior minister and as a member of the committee that selects judges.

Noam Sohlberg, a West Bank settler, was the only overtly conservative justice sitting on the bench at the beginning of Ms. Shaked’s tenure.

Though the court is now more ideologically diverse, significant parts of Israeli society remain underrepresented on the body, including Jews of Middle Eastern descent. There is just one Muslim justice, Khaled Kabub, from the Palestinian Arab minority that makes up 20 percent of Israel’s population.

At the hearing on Sept. 12, the court is scheduled to hear multiple petitions by rights groups and individuals calling it to strike down the law passed by Parliament in July. That legislation cancels the court’s ability to use the somewhat vague and subjective standard of reasonableness to overturn government decisions and appointments.

Critics of the government say that reasonableness is an essential tool for a court that is the only check on majority rule in a country with one house of Parliament, a figurehead president and no formal, written constitution. Even Justice Sohlberg, who has argued in favor of curtailing the use of the reasonableness standard, has balked at its being restricted by legislation.

By convening all 15 justices, experts say, Chief Justice Hayut cannot be accused of weighting the outcome by exclusion. A regular panel is composed of three justices, though they sometimes sit on expanded panels.

The case is particularly fraught because the Supreme Court, in making a decision on the law passed in July, will be ruling on an amendment to one of the Basic Laws that have quasi-constitutional status in Israel. The court has never intervened in a Basic Law before but says it has the right to do so. The government says it does not.

The battle over the judicial overhaul plan will probably be Chief Justice Hayut’s last action before she ends her term. In mid-October she will turn 70, the mandatory retirement age for justices.

As she and other judges retire, at least four Supreme Court slots will open up over the next few years, and Yariv Levin, the current justice minister and a main architect of the overhaul, appears determined to give the government more say on the replacements. Mr. Netanyahu has confirmed that he will push ahead in the fall with a law to change the way that judges are selected.

Critics view that as the most polarizing piece of the overhaul, saying that it would politicize the court and essentially turn Israel into an autocracy.

For decades, the nine-member judicial selection committee has been made up of two government ministers, two lawmakers (usually one from the ruling coalition and one from the opposition), three Supreme Court justices and two lawyers from the Israel Bar Association, giving legal professionals an edge over the politicians. Supreme Court appointments require a special majority of seven.

Ms. Shaked said that judicial reform should be carried out only by consensus and that the damage wrought by the government’s effort to push through the overhaul outweighed any benefits.

“I’d say, ‘Use the existing system and get in one or two more conservatives without breaking the country apart,’” she said.

Myra Noveck contributed reporting from Jerusalem, and Gabby Sobelman from Rehovot, Israel.