JERUSALEM — His army nickname was Benny Huta, a Hebrew pun that translates as “Benny Chill.” And in his first-ever run for office, Benny Gantz never let anyone see him sweat, no matter the live-television gaffes, embarrassing leaks or rumors about personal improprieties that occasionally tripped up his candidacy for prime minister.
His centrist, flag-hued Blue and White party did better than any new Israeli faction ever had before, earning more than a million votes and around 35 seats in Parliament on Tuesday. That was the same as Prime Minister Benjamin Netanyahu’s Likud party did, though smaller right-wing factions tilted the election sharply to the incumbent.
It was quite a showing after just a few months in politics. Now, Mr. Gantz, a three-star general and former Israeli military chief, is laying in for a siege against Mr. Netanyahu that his allies expect could last into 2020.
“Nothing will drive us from our goal,” Mr. Gantz said Wednesday night.
With Mr. Netanyahu expected to form the most right-wing and ultra-Orthodox government Israel has ever seen, it will probably be a bumpy road even before a coalition is formed.
The issue of military conscription is likely to put secular and ultra-Orthodox parties in his coalition immediately at odds. Mr. Netanyahu’s expected indictment is looming, and may come as soon as July. And then there is the wild card of a Trump administration peace plan — one that some of his coalition partners could well find unacceptable — that could be announced at any time.
On Wednesday, Mr. Gantz’s partner in Blue and White, Yair Lapid, offered a warning to Mr. Netanyahu and the Likud: “We are going to make your lives miserable.”
“We will turn the Knesset into a field of battle,” he said. “And we will do one more thing: We will show the citizens of Israel how a real, true alternative looks.”
Yet as a candidate, Mr. Gantz, with his hooded eyes, mild manner and above all his willingness to let Mr. Lapid take over as prime minister after two years of their term, left many voters wondering if he had the fire in the belly — or as Israelis call it, the “knife in the teeth” — to defeat Mr. Netanyahu.
And on Wednesday, he spoke less militantly than Mr. Lapid of his plans as opposition leader.
“I have come not for power, I came for a goal,” he said. “I came for Israeli society. I came from a place of great love for Israeli society and a place of great love for the country.”
For Mr. Gantz, merely holding his seven-week-old party together would be its own achievement. Blue and White comprises left, centrist and right-wing politicians who were able to get behind a noncontroversial platform, but have not yet had occasion to fight over actual legislation.
It is a merger of three parties: Mr. Gantz’s Israel Resilience, itself a fledgling party; a small, more hawkish faction led by Moshe Yaalon, a former defense minister and army chief who is a veteran of Mr. Netanyahu’s conservative Likud party; and Yesh Atid, an established centrist party led by Mr. Lapid, a former finance minister. A third army chief, Gabi Ashkenazi, also occupies a top leadership spot.
All four put their egos aside for the campaign — though that was with an outcome in mind that did not involve sitting in the opposition.
Blue and White will also have to resist what are certain to be the wily Mr. Netanyahu’s efforts to divide and conquer, and there are several visible seams Mr. Netanyahu could test. The prime minister could woo the half-dozen right-wing members with legislation they want, straining the discipline of their new party. He could also offer the defense ministry to Mr. Gantz or either of the other retired generals.
Mr. Lapid, who built Yesh Atid into a national organization in pursuit of his own ambition to become prime minister, may see himself as a more capable leader of the opposition in Parliament than Mr. Gantz. And other party members with important jobs like Avi Nissenkorn, the chairman of the labor federation Histradut — Israel’s A.F.L.-C.I.O. — may grow impatient as lawmakers in the minority.
Mr. Lapid, the most seasoned spokesman of the bunch, tackled those doubts head-on. “I see an ensemble,” he said. “They will try to break us up. They will not succeed.”
Analysts have speculated that a Trump peace plan might prompt Mr. Netanyahu to appeal to Mr. Gantz to form a broad national unity government. But Yoaz Hendel, a former Netanyahu spokesman who was No. 2 in Mr. Yaalon’s Telem Party, said Mr. Gantz would have little trouble resisting such advances.
“I don’t see any reason to help Netanyahu with a Trump plan,” Mr. Hendel said. “If he has his own coalition, I offer him to do it by himself. I don’t see any reason to give him a defense shield.”
Mr. Gantz ran on a slate dedicated to integrity, clean government and repairing the system. The party offered voters something less than a revolution. But the mild tones of its campaign appealed to a widespread yearning among many Israelis to return to an older, possibly idealized Israel, where leaders were modest to the point of frugality and put the good of the country before party or personal interests.
Mr. Gantz spoke of being a prime minister for all Israeli citizens, Jews and non-Jews alike, itself a noteworthy statement in the current environment. But he ruled out forming a coalition with the leaders of Arab parties. And to appeal to as many voters as possible, he espoused no firm policy positions and said little about fundamental issues like the Israeli-Palestinian conflict.
Born to parents who survived the Holocaust, became Zionist pioneers and helped found a farming community in the early years of the state, Mr. Gantz speaks English with a thick Israeli accent and is understated to the point of appearing bashful.
He lives unostentatiously in Rosh Haayin, in central Israel, in a suburban neighborhood of neat, semidetached homes built for army officers. His hobbies include attending communal singalongs of Hebrew oldies like “Hareut,” his favorite, a haunting rendition of a poem by Haim Gouri, who died last year. The song memorializes the comradeship of those who fought in the 1948 war surrounding Israel’s creation.
Mr. Gantz grew up in a religious family in Kfar Ahim, a cooperative agricultural village near the border with Gaza. His father, Nahum, was from Romania. His mother, Malka, who was Hungarian, was liberated from the Bergen-Belsen concentration camp.
In an emotional address at the launch of the Blue and White alliance in February, Mr. Gantz related that 75 years ago, Mr. Lapid’s father, Tommy, and Mr. Gantz’s mother shared a room together in the Jewish ghetto in Budapest.
“And now we stand here before you, the son of Tommy, of blessed memory, and the son of Malka, of blessed memory, sharing one stage,” he said. “Because the state of Israel is a miracle.”
Mr. Gantz’s parents were friends with Shimon Peres, who served as Israel’s prime minister and later as president, said Ayelet Frish, a strategic consultant who worked for Mr. Peres, who died in 2016.
“He’s as straight as an arrow,” she said of Mr. Gantz. “He is not quick to take decisions, but he knows how to take them.”
In his long career, Mr. Gantz spent two years as Israel’s military attaché in Washington, and once rode across the United States on a Harley-Davidson. He retired from the Israel Defense Forces in 2015.
Fania Oz-Salzberger, the daughter of Amos Oz, the Israeli writer and peace advocate who died last year, said her father and Mr. Gantz “were respectful friends, though Gantz is politically right of Oz.”
“Gantz often visited my ill father,” she said on Twitter. “They talked a lot. And father told us: ‘A good leader must understand human nature, read the map, have courage and compassion. That’s Gantz.’”