Isaac Herzog, who was elected head of the Labor Party in November, said that his ultimate goal is to replace Netanyahu.
But he said he would back the prime minister if he genuinely pursues peace with the Palestinians and offered a political "safety net" should Netanyahu's right-leaning coalition rebel in case of substantial progress in recently restarted peace talks.
As opposition leader, Herzog meets privately at least once a month with Netanyahu. Asked about the talks, Herzog said he has found a leader who "understands the real risks" Israel faces without a peace deal. At the same time, he said wasn't sure Netanyahu "has the guts to do it."
Under heavy American pressure, Israelis and Palestinians began talks last year July after years of stalemate. So far, there has been no outside sign of progress and the talks have been characterised by mutual mud-slinging.
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The sides have set an April target date for a "framework" agreement.
The Palestinians seek the West Bank, east Jerusalem and Gaza Strip, territories captured by Israel in 1967, for an independent state.
He also has insisted that the Palestinians recognise Israel as the Jewish homeland, a condition they say would undermine the rights of Palestinian refugees and Israel's own Arab minority.
The Palestinians have generally accused him of stalling and negotiating in bad faith, citing continued Israeli settlement construction in the West Bank and east Jerusalem.