Likud MK Gideon Sa'ar officially launched his campaign on Monday in a bid to unseat Prime Minister Benjamin Netanyahu as head of the party amid rising support.

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Netanyahu is Israel's longest-serving leader, but he's been weakened by a corruption indictment that may force him out of office, as well as back-to-back failures to form a government this year.

Gideon Sa'ar speaking at his campaign launch rally ( Photo: Moti Kimchi )

Although Sa'ar is still a decided underdog to the embattled prime minister, he seems to be gaining traction ahead of the Dec. 26 vote among the party faithful. Sa'ar's leadership bid marks the first serious internal challenge to Netanyahu in his decade-plus in power.

Praising Netanyahu while calling for a change in leadership, Sa'ar said Monday that the prime minister "brought us to power four times, but the writing is on the wall. There won't be a fifth time."

A former aide and senior Cabinet minister under Netanyahu, Sa'ar has long been considered a rising star in Likud and a potential future heir. But while others are patiently waiting for Netanyahu to step down on his own, Sa'ar has been the only one who has dared to take him head-on.

Benjamin Netanyahu ( Photo: AP )

Netanyahu faces charges of bribery, fraud and breach of trust in three corruption cases in which he is accused of trading legislative or regulatory favors in exchange for lavish gifts or favorable media coverage. He denies wrongdoing and has waged an angry campaign against the media and law enforcement officials he said are bent on ousting him from office.

Outwardly, Likud members have strongly supported their leader and joined in denouncing the alleged "coup" of the liberal elites against him.

But Sa'ar's burgeoning insurrection has begun to reveal some cracks.

Likud activists at the rally supporting both Netanyahu and Sa'ar ( Photo: Moti Kimchi )

Around 500 party activists attended the launch of the former education minister's campaign to unseat Netanyahu, held in the Tel Aviv suburb of Or Yehuda. Sa'ar's campaign has already been buoyed by support from a half-dozen Likud lawmakers. That includes the powerful chairman of the party's executive body. Sa'ar, in a jab at Netanyahu's inability to form a government, took the podium flanked by banners with his slogan: "Only Sa'ar Can."

"A vote for me will ensure Likud rule and the formation of a new government headed by us," Sa'ar told supporters. "A vote for Netanyahu is a vote for the next head of the opposition."

Gideon Sa'ar and Likud Central Committee head Haim Katz ( Photo: Moti Kimchi )

While Netanyahu's various opponents across the political spectrum have called on him to resign because of his legal woes, Sa'ar has continued saying the party needs a new leader because Netanyahu has been unable to form a stable coalition government.

Recent polls show that with Saar as the leader, Likud would make a more powerful bloc with its natural ultra-Orthodox and nationalist allies. Sa'ar would also be in an easier position to create a national unity government with the centrist Blue and White party if, as expected, the upcoming March election produces a deadlock like the previous two rounds have.

Blue and White have ruled out serving with Netanyahu because of his indictment.

Even amid the political disarray, Sa'ar's challenge is a risky maneuver in a party that fiercely values loyalty and has had only four leaders in its 70-plus-year history. Netanyahu himself has denounced Sa'ar as "subversive."

A frame from a video made by pro-Netanyahu Likud activists portraying Sa'ar as a terrorist ( Photo: Screenshot )

Earlier Monday, Sa'ar challenged Netanyahu to a policy debate in a video released on social media.

"I am waiting for you, Mr. Prime Minister, at any time and any place you choose," he said.

Netanyahu is desperate to stay on as prime minister as he girds up for his trial. If he is re-elected, Netanyahu could use the post-election coalition negotiations to offer potential partners coveted Cabinet posts in exchange for their support for immunity.