TEL AVIV—Israel’s national election is becoming particularly vicious and personal as campaigning enters its final week, highlighting how the battle for Israel’s premiership remains closely contested.

With a small advantage in polls showing he can build a majority coalition, Prime Minister Benjamin Netanyahu has launched an offensive against Benny Gantz, a former general whose centrist coalition poses a stiff challenge. Longtime Israeli political observers say Mr. Netanyahu’s campaign has been brutal even by the standards of Israeli elections, which are often slugfests.

In the past week, Mr. Netanyahu has accused his opponent of being mentally unfit to be defense minister, let alone prime minister. The prime minister’s Likud party has said Mr. Gantz has “lost it,” running a series of ads that use gaffes by the first-time politician to create a picture of a man unfit for office.

One ad features Mr. Gantz repeating the words “totally stable” over and over, as the frame zooms in on his bulging eyes while scratchy, high-pitched string music from “Psycho” plays.

The attacks mark a reversal in Mr. Netanyahu’s tone about Mr. Gantz, who served the prime minister as chief of staff of the Israeli military in 2011 and was once a key security ally.