JERUSALEM — Ehud Barak, a former Israeli prime minister and longtime critic of Prime Minister Benjamin Netanyahu, announced Wednesday that he was forming a new political party to run in the September election.

Mr. Barak, 77, who is also a former defense minister and a decorated military chief, has long been rumored to be planning a comeback. He has used Twitter to denounce Mr. Netanyahu in recent years for his right-wing policies and a corruption investigation that could lead to the prime minister’s indictment.

Israel will hold a do-over election in September because Mr. Netanyahu failed to form a viable coalition after the April ballot.

In a televised news conference on Wednesday, Mr. Barak challenged Mr. Netanyahu’s propriety to continue in office, accusing him of having precipitated the last election for the purpose of trying to disrupt legal proceedings against him.