JERUSALEM — A black curtain went up a few months ago near Prime Minister Benjamin Netanyahu’s official residence on Jerusalem’s leafy Balfour Street. It screened pesky protesters from Mr. Netanyahu’s view — and prevented the public from seeing lawyers and detectives come and go as criminal investigations of the prime minister intensified.

Now, with one of Mr. Netanyahu’s closest former aides having turned state’s witness in two cases involving suspicions of bribery, fraud and breach of trust, Israelis across the political spectrum are trying on the idea of the curtain coming down on Mr. Netanyahu’s durable political career.

For the past eight years, Mr. Netanyahu has dominated the Israeli political scene and become nearly synonymous with the state on the world stage. Long called “the magician” for his survival skills, he has quashed rivals from right and left despite never enjoying particular popularity in the street.

Yet Mr. Netanyahu, the longest-serving prime minister since Israel’s first, David Ben-Gurion, suddenly appears not so invincible after all.