JERUSALEM — One of Prime Minister Benjamin Netanyahu’s closest and longest-serving aides appeared ready to incriminate him on Wednesday after agreeing overnight to become a government witness, the latest twist in a spiraling graft scandal that seemed to dim Mr. Netanyahu’s legal and political chances of survival almost by the hour.

The fast-moving police inquiry into whether Mr. Netanyahu, already battling separate bribery allegations, had provided official favors to Israel’s largest telecommunications company, Bezeq, in exchange for fawning coverage on the company’s online news site prompted one member of the prime minister’s party to ask him to step aside and opposition politicians to call for early elections.

Mr. Netanyahu, who insists he has done nothing wrong, has faced corruption allegations periodically almost since first becoming prime minister in 1996. But the latest — with its suggestion of political payoffs to a company that bills ordinary Israeli voters every month — could prove the most damning. And as the revelations mounted, one on top of another like a tottering tower, Israelis expressed increasing doubt about Mr. Netanyahu’s ability to maintain his grip on power.

“It was like watching a police car chase in pursuit of a robber on one of America’s endless highways,” Sima Kadmon, a columnist, wrote in Wednesday’s Yedioth Ahronoth of the flurry of events of the day before. “Riveting hours of dramatic and fateful revelations that are going to change not only the life of the man behind the wheel, but the face of our country.”