A senior political ally of Prime Minister Ehud Olmert's said Monday that Israel should release its most prominent Palestinian prisoner - a man convicted in fatal attacks on Israelis - in a bid to prop up Palestinian President Mahmoud Abbas. Environment Minister Gideon Ezra, former deputy head of the Shin Bet, is the second senior Israeli official to recently advocate the release of Marwan Barghouti of Abbas's Fatah movement. Barghouti is the most popular leader in the Palestinian territories, and is widely regarded as the only figure able to unify clashing Palestinian factions, rein in militants and get peacemaking with Israel moving again. In 2004 Marwan Barghouti was convicted for five murders and one attempted murder as leader of the Aksa Martyrs Brigades. He was sentenced to five life-terms behind bars. "If we want to blunt Hamas' capabilities ... and if we ultimately want a civil rather than a religious government like those taking shape across the Arab world, we have to make a contribution," Ezra told Army Radio, in defense of freeing Barghouti. "I think it could definitely help Abbas," Ezra added. Abbas and his Fatah loyalists are engaged in an increasingly deadly power struggle with the Islamic Hamas, which rejects Israel's right to exist and unseated Fatah in elections last year. The infighting has weakened Abbas as he tries to re-launch long-stalled peace talks with Israel, which considers him a legitimate negotiating alternative to Hamas. Olmert spokeswoman Miri Eisin said the release of Barghouti - who is serving five life sentences in the murders of four Israelis and a Greek monk - was "not on the agenda." But Ezra, a member of Olmert's Kadima Party, said Israel has freed "much worse murderers" in the past. Several weeks ago, Deputy Defense Minister Ephraim Sneh (Labor) also championed Barghouti's release. Hamas has demanded a large-scale prisoner release, including Barghouti, in exchange for Cpl. Gilad Shalit, an Israeli soldier captured by Hamas-linked militants in June.