Veteran Israeli politician Gideon Saar has launched his bid to unseat prime minister Benjamin Netanyahu as head of the ruling Likud party.

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

Saar’s leadership bid marks the first serious internal challenge to Netanyahu, who has served more than 10 years in office. Though Saar is still a decided underdog to the embattled prime minister, he seems to be gaining traction ahead of a party vote on Thursday next week.

A former aide and senior cabinet minister under Netanyahu, Saar 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, Saar has been the only one who has dared to take him head on.

Netanyahu faces charges of bribery, fraud and breach of trust in three corruption cases in which he is accused of trading legislative or regulatory favours in exchange for lavish gifts or favourable 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 Saar’s burgeoning insurrection has begun to reveal some cracks.

Around 500 party activists attended the launch of the former education minister’s campaign to unseat Netanyahu, which has been buoyed by support from a half-dozen Likud lawmakers. That includes the powerful chairman of the party’s executive body. Saar, in a jab at Netanyahu’s inability to form a government, took the podium flanked by banners with his slogan: “Only Saar Can!”

While Netanyahu’s various opponents across the political spectrum have called on him to resign because of his legal woes, Saar has kept saying the party needs a new leader because Netanyahu has been unable to form a stable coalition government. He has said the prime minister is unlikely to be able to again, if given another chance.

Recent polls showed that with Saar as leader, Likud would make a more powerful bloc with its natural ultra-Orthodox and nationalist allies. Saar 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, Saar’s challenge is a risky manoeuvre in a party that fiercely values loyalty and has had only four leaders in its 70-plus-year history. Netanyahu himself has denounced Saar as “subversive”.

Earlier on Monday, Saar 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.