Benjamin Netanyahu, leader of the conservative Likud Party, was invited by Israel’s president, Shimon Peres, on Friday to take the lead in assembling the next government. Whatever form that government takes, Mr. Netanyahu, 59, is widely expected to return as prime minister a decade after the last government he led fell apart.

In a brief but statesmanlike speech at the presidential residence on Friday afternoon, Mr. Netanyahu accepted the mandate and immediately called on the centrist Kadima Party, led by Tzipi Livni, and the center-left Labor Party, led by Ehud Barak, to join him in a unity government. He said national unity was necessary in order for Israel to contend with the formidable challenges ahead.

“Let us unite to secure the future of the State of Israel,” Mr. Netanyahu said, adding that he wanted to discuss the possibility of forming a broad government “for the good of the people and the state.”

Mr. Netanyahu and Ms. Livni have agreed to meet on Sunday, but the negotiations between them are likely to be tough and the chances of success are unclear.

Ms. Livni, the current foreign minister and Mr. Netanyahu’s main rival for the premiership, has so far refused the idea of joining a government led by Mr. Netanyahu and including several ultra-orthodox and far-right parties. Committed to the peace process with the Palestinians, she has said she would rather go into the opposition than serve as a fig-leaf for a coalition of the right.

Mr. Barak, whose Labor Party fared badly in the elections, has already said he would heed the will of the people and head into the opposition.

But in his plea for unity, Mr. Netanyahu pointed to the existential threat to Israel that would be posed by a nuclear Iran and the global economic crisis that he said could cost hundreds of thousands of Israelis their jobs.

Such major challenges, Mr. Netanyahu said, required “a new approach” of unity and of “joining hands.” Striking a more positive and conciliatory tone, he said the goal was to seek “peace with our neighbors and unity among ourselves.”

A broad government joined by the center and left would likely promote a more pragmatic agenda and avoid friction with Israel’s most important ally, the United States.

Mr. Netanyahu will have up to six weeks to put together a governing coalition. He was tapped for the premiership after he gained the endorsement of 65 members of the 120-seat Parliament, from religious parties and those on the far right.

While Ms. Livni’s Kadima narrowly won the Feb. 10 elections, it failed to muster the support of a majority in the Parliament, a prerequisite for forming a governing coalition. Ms. Livni gained the endorsement only of Kadima’s 28 legislators.

In the last few days, many here have predicted that Mr. Netanyahu would be left with no choice but to form a narrow government with those he has termed his “natural partners,” parties representing the ultra-orthodox and the right.

After a private meeting earlier Friday with Mr. Peres, who has also been urging national unity, Ms. Livni said that the coalition taking shape lacked political vision and that “a broad coalition has no value if it does not lead the way.”

But in his speech on Friday, Mr. Netanyahu indicated that he wanted to embark on a new track. He said he would meet first with Ms. Livni and Mr. Barak, and made no mention of the parties that endorsed him — groups like Avigdor Lieberman’s ultra-nationalist Yisrael Beiteinu, which came third in the elections, or the ultra-orthodox Shas.

Rina Mazliah, a political commentator with Israel’s Channel 2 News, said that Mr. Netanyahu had changed direction and that his call for unity based on national responsibility might make it difficult for Ms. Livni to refuse.

Nevertheless, Mr. Netanyahu’s vision of unity seemed far from assured.

Leading members of Mr. Barak’s once-dominant Labor Party, which won a mere 13 seats in the new Parliament, have said the party must spend time rebuilding itself in the opposition.

Moreover, the parties to any coalition would have to agree on basic government guidelines.

Ms. Livni has staked her political career on promoting negotiations with the more pragmatic, Western-backed Palestinian leadership for a two-state solution. Mr. Netanyahu says he wants to promote “economic peace” in the West Bank but has remained deliberately vague about any long-term political solutions.

Ms. Livni noted on Thursday that Mr. Netanyahu “meanwhile refuses to talk about a two-state solution.”

Mr. Netanyahu acknowledged on Friday that there were serious political differences, but he said that given the momentousness of the hour he believed it was possible to “find a common path to reach the country’s goals.”

A narrow government would be less stable, with Mr. Netanyahu having to balance the often competing demands of small parties. A right-wing agenda would also set Israel on a possible collision course with the new administration in the United States, which has pledged an active and aggressive pursuit of peace.

Shalom Yerushalmi, a columnist in Friday’s Maariv newspaper, described such a government as Mr. Netanyahu’s “nightmare.”

“The narrow government he formed in 1996 fell apart in stages,” Mr. Yerushalmi noted. “Netanyahu swore that he would not make a narrow government again, and would never again be the prime minister of half the people.”