JERUSALEM (Reuters) - Former military chief Benny Gantz received an official mandate on Wednesday to try to form Israel’s next government, but with no easy path to ending Prime Minister Benjamin Netanyahu’s long hold on power.

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After inconclusive elections in April and September, Gantz’s nomination marked the first time since 2008 that someone other than Netanyahu, 70, has been asked by Israel’s president to build a ruling coalition.

Head of the centrist Blue and White party, Gantz will have 28 days to complete the task assigned by President Reuven Rivlin in a televised ceremony. Failure could lead to a new election, the third within a year.

“Everyone expects us to bring the political chaos to an absolute end,” Gantz said, accepting the nomination.

Rivlin gave Netanyahu the chance to form a government first. But the prime minister, who leads the right-wing Likud party, said on Monday he was abandoning the effort, opening the way for Gantz, his strongest rival.

Replacing even a weakened Netanyahu after a decade in office could prove difficult without a significant shifting of political alliances.

Gantz, 60, has so far spurned Netanyahu’s offer to Blue and White to join Likud in a “national unity” government, citing looming indictments against the prime minister in three graft cases. Netanyahu denies the allegations.

Gantz, who headed Israel’s military from 2011 to 2015,, has the endorsement of only 54 lawmakers - seven short of a parliamentary majority that neither he nor Netanyahu could manage in either last month’s vote or the ballot in April.

Gantz has also balked at including ultra-Orthodox Jewish parties - some of Netanyahu’s traditional allies - in his administration.

In his acceptance speech, Gantz spoke of “national reconciliation” but also described a future government in terms that could exclude the ultra-Orthodox.

“I promised to establish a liberal unity government, and that, I intend to do,” he said, using political shorthand for an administration that pursues a secular agenda, such as a wider opening of businesses on the Jewish Sabbath.

Differences also emerged in coalition talks over Rivlin’s proposal for a rotating premiership, including a possible leave of absence for Netanyahu should he be indicted. Israel’s attorney general is expected to announce his decision on charges by the end of the year.

Negotiators from Blue and White and Likud planned to meet again on Thursday. In a statement, Likud repeated its call for a broad coalition government, a nod to inclusion of the ultra-Orthodox.

Netanyahu remains caretaker premier until a new government is established. He has been prime minister consecutively since 2009, after a first term from 1996 to 1999, making him Israel’s longest-serving leader.