During an aggressive speech at the United Nations on Tuesday, Prime Minister Benjamin Netanyahu of Israel used sarcasm and combative words to portray Iran's new president, Hassan Rowhani, as a smooth-talking charlatan, one who is determined to continue building a nuclear-weapons arsenal.

Netanyahu called Mahmoud Ahmadinejad, the previous Iranian president, "a wolf in wolf's clothing" and Rowhani "a wolf in sheep's clothing."

Netanyahu has legitimate reasons to be wary of any Iranian overtures, as do the United States and the four other major powers involved in negotiations over Iran's nuclear program. But it could be disastrous if Netanyahu and his supporters in Congress were so blinded by distrust of Iran that they exaggerate the threat, block President Obama from taking advantage of new diplomatic openings and sabotage the best chance to establish a new relationship since the 1979 Iranian revolution sent American-Iranian relations into the deep freeze.

Netanyahu has hinted so often of taking military action to keep Iran from acquiring a nuclear weapon that he seems eager for a fight. He did it again at the United Nations, warning that Israel reserved the right to strike Iran's nuclear facilities if it deemed that Iran was close to producing nuclear weapons.

Both Obama and Rowhani have hard-line domestic audiences and allies that they will need to consider and cajole as they undertake this effort to resolve the nuclear dispute and develop a new relationship. For Obama, that means working closely with Israel and helping Netanyahu see that sabotaging diplomacy, especially before Iran is tested, only makes having to use force more likely. That would be the worst result of all.

FROM AN EDITORIAL IN THE NEW YORK TIMES