JERUSALEM – The Israeli defense minister, Ehud Barak, announced Monday that he would soon “leave political life,” after a half-century career in the military and government that included two years as prime minister.

Coming days after the end of a weeklong air blitz on the Gaza Strip and eight weeks before Israelis head to the polls, Mr. Barak’s move is the latest to show the disarray in Israel’s center-left bloc. Though he formed a close partnership with the right-leaning prime minister, Benjamin Netanyahu, particularly on the Iranian nuclear threat, Mr. Barak was a longtime leader of the liberal Labor Party, and now heads the tiny Independence faction.

Polls have suggested for months that Independence might not win enough votes for even a single seat in the Parliament, so some observers saw Mr. Barak’s decision as a way to avoid such an embarrassment, instead walking away amid praise for a Gaza operation that killed high-ranking Hamas militants and significantly reduced their arsenal of long-range rockets. He did not address the question of whether he would accept what is known as a “personal appointment” to serve a new government as defense minister despite sitting out the elections.

“I came to this decision not without misgivings but in the end a whole heart,” Mr. Barak, 70, said at a news conference in Tel Aviv. “I would be concealing the whole truth if I did not say that the warmth that I feel from the public — favorable coverage from some of you, in recent days — wasn’t nice. As someone who has not been indulged in that way usually, I know how to appreciate this and rejoice in it.”

Mr. Barak, who has been well regarded as defense minister but is not a personally popular figure, was never considered a major factor in the shifting alliances for the coming elections. His announcement disrupted the swirling speculation over the plans of Tzipi Livni, the former foreign minister and head of the centrist Kadima Party, who is expected to announce that she is re-entering politics on a new ticket.

Ms. Livni was courted both by the Labor Party and by another new centrist party, Yesh Atid, or There Is a Future, run by the former journalist Yair Lapid, but recent reports in the Israeli news media suggest that she has decided to go it alone. Ms. Livni also was among those who reportedly tried to persuade Shimon Peres, 89, currently filling the largely symbolic post of Israel’s president, to make a late-in-life comeback and challenge Mr. Netanyahu.

Ehud Olmert, the former prime minister who has spent recent years battling corruption charges, has also apparently decided against a comeback bid right now, leaving Ms. Livni, Mr. Lapid and the current heads of Kadima and Labor to battle it out for the dwindling center-left electorate. Analysts say this landscape is unlikely to change the fundamental dynamic in which right-wing and religious parties win a majority of Parliament seats, and Mr. Netanyahu is widely seen as the most likely to form a new government.

An article about Ms. Livni’s plans in the newspaper Maariv on Monday — published before Mr. Barak’s surprise withdrawal — carried the headline “Shooting the Left in the Foot.”

“The ratio between the blocs is to the center-left’s disadvantage,” wrote the author, Mazal Mualem, adding that the recent military operation “strengthened the right even more and made attitudes more extreme.” Ms. Livni “is aiming her fire at Netanyahu, but in practice is taking seats from her own bloc,” the article said.