JERUSALEM — A former top aide to Prime Minister Benjamin Netanyahu of Israel turned state’s witness on Monday, the third close associate to agree to testify against him in a corruption scandal that threatens to end his career.

The deal came after days of feverish speculation in Israel that Mr. Netanyahu, beleaguered by police investigations and facing possible bribery charges, might call a snap election in hopes of a quick validation of public support.

News of the plea deal broke as Mr. Netanyahu was in Washington to meet with President Trump, diminishing any hope the Israeli leader may have held that his role on the international stage would overshadow the scandals back home.

Speaking to Israeli reporters in Washington, Mr. Netanyahu called the allegations “nonsense” and played down the importance of aides who have become government witnesses against him. But he declined to speak on the record about the case in any detail.