JERUSALEM — Ehud Barak, who over a half-century career became Israel’s most decorated soldier and held the nation’s trifecta of top positions — chief of staff of the military, prime minister and, since 2007, defense minister — announced Monday that he would soon “leave political life,” withdrawing from elections scheduled for Jan. 22.

“I came to this decision not without qualms, but in the end, with a whole heart,” Mr. Barak, 70, said at a morning news conference in Tel Aviv. “I feel I have exhausted the political vocation,” he added. “Change in positions of power is a good thing.”

Mr. Barak could yet be appointed to another term. But his withdrawal from the campaign, coming on the heels of an eight-day air blitz on the Gaza Strip and eight weeks before Israeli elections, highlighted the disarray in Israel’s center-left bloc, where political operatives have been scrambling to organize a credible challenge to Prime Minister Benjamin Netanyahu. After weeks of deliberation, two other former prime ministers — Ehud Olmert, who has spent recent years battling corruption charges; and the 89-year-old Shimon Peres, now in the largely symbolic role of president — have both apparently decided against making comeback bids.

Tzipi Livni, the former foreign minister and head of the centrist Kadima Party, is expected to announce her own re-entry into politics on Tuesday, forming a new party after rebuffing offers to help lead both the liberal Labor Party and a new centrist party called There Is a Future. But analysts say that is unlikely to shift the ground much in terms of taking votes away from the right-wing and religious parties now in Mr. Netanyahu’s coalition, and that he is the strong favorite to form the next government.