JERUSALEM — Political cartoons depict flames licking at the foundations of the fortresslike household of Prime Minister Benjamin Netanyahu. Commentators say the noose is tightening around his neck.

For months, Mr. Netanyahu has been under investigation in two separate, leak-ridden graft cases involving illicit gifts from wealthy friends and back-room dealings with a local newspaper magnate in a bid for favorable coverage. The latest blow came just before the weekend, when the Israeli police signed a state’s witness deal with Ari Harow, Mr. Netanyahu’s former chief of staff and once one of his closest confidants.

A day earlier, in a legal document pertaining to the negotiations with Mr. Harow, the police said in writing, for the first time, that Mr. Netanyahu was suspected of bribery, as well as fraud and breach of trust.

In light of these developments, analysts say, it appears likely that Israel’s longest-serving prime minister after David Ben-Gurion will ultimately face charges, injecting his fourth term with a new level of turbulence and intrigue. The question now, his critics say, is how long he can stave off what they view as his looming political demise.