Israeli Defense Minister Ehud Barak announced on Monday that he was leaving politics and would not run in parliamentary elections in January, but would stay in office until then. (Bernat Armangue/AP)

Israeli Defense Minister Ehud Barak abruptly announced Monday that he is leaving politics and will not run in parliamentary elections in January, quashing speculation that he might unite with other centrist candidates to challenge Prime Minister Benjamin Netanyahu.

Barak, who declared that he will step down when a new government is formed after the elections, said he felt he had “exhausted my involvement in politics, of which I was never particularly enamored.”

Long unpopular as a political candidate, Barak faced poor election prospects for his tiny Independence faction despite a modest bump in the polls following Israel’s recent military offensive in Gaza against the Islamist group Hamas.

By announcing his departure after that campaign, which boosted his approval ratings as defense minister, Barak appeared positioned for a potential return to the job as an outside cabinet appointment after the Jan. 22 elections. He was noncommittal Monday when asked if he might return to the cabinet if he were invited back to serve as defense minister.

“I will answer [that question] on January 22,” he said, adding that “these issues were not asked or discussed.”

Barak’s announcement came as a surprise after recent reports that he was negotiating an alliance with Tzipi Livni, the former head of the opposition Kadima party, to lead a centrist bloc that could take on Netanyahu. Livni is expected to announce her plans this week.

Barak told a news conference that he had decided “to leave political life and not run in the elections for the coming Knesset,” Israel’s parliament. A former prime minister and army chief of staff, Barak, 70, said he wanted to devote more time to his family and “allow others to assume senior positions in Israel.”

Netanyahu, who is favored to win another term as prime minister, issued a statement saying that he respects Barak’s decision and that he “very much appreciates his contribution, over many years, to the security of the state.”

After working in tandem with Netanyahu to raise the alarm about Iran’s nuclear program, Barak appeared in recent months to be trying to set himself apart from the prime minister, highlighting his own role in strengthening security ties with Washington.

When Netanyahu publicly criticized the Obama administration in September for not drawing a “red line” regarding Iran’s nuclear efforts, Barak criticized the move, saying such disagreements should be resolved “behind closed doors.” He later floated the idea of unilaterally withdrawing from much of the West Bank while leaving large settlements under Israeli control, an idea that has been rejected by Netanyahu.

Shlomo Avineri, a political science professor at the Hebrew University of Jerusalem, noted that while Barak had announced a departure from politics, he had not ruled out a return to defense issues, making him a potentially desirable addition to any future government, whether led by Netanyahu or his challengers.

“I think he positioned himself as the responsible adult,” Avineri said. “For the first time in many years, people are courting him, which is a novelty, because he has not been particularly popular on the political scene.”

Both Livni and Shelly Yachimovich, the leader of the Labor Party, paid tribute to Barak, expressing regret over his decision.

Barak’s party, which broke away from Labor, previously struggled in public opinion polls to cross the electoral threshold for a seat in parliament, and some polls after the Gaza offensive showed it reaching a mere four seats in the 120-member legislature.

By cutting himself loose from that political millstone, Barak appeared to be keeping his options open. Asked by a reporter when he would be making a comeback, Barak hesitated before replying: “I have no answer.”

“He’s now waiting on the sideline, probably for some offers,” said Shmuel Sandler, an expert on politics and foreign policy at the Begin-Sadat Center for Strategic Studies at Bar-Ilan University. “Israel is not like America. You have so many comebacks.”