The Israeli prime minister has warned that a deal under discussion on Iran's nuclear programme could "pave its way to the bomb", rather than block it.
In a speech to US Congress punctuated by standing ovations, Benjamin Netanyahu depicted Iran as a "threat to the entire world".
Talks on Iran's nuclear programme are nearing a critical late-March deadline for an outline agreement to be reached.
The speech comes just two weeks before a closely fought election in Israel.
Mr Netanyahu insisted that he was not trying to meddle in internal US politics.
The White House had criticised his plan to address Congress - at Republican invitation, without consulting the White House - saying the Israeli leader was trying to swell US opposition to any Iran deal.
Following the speech, US President Barack Obama told reporters he had seen nothing new in the speech transcript and that no viable alternative to preventing Iran from obtaining a nuclear weapon had been presented.
The US, UK, France, Germany, Russia and China are seeking to reach agreement to curtail Iran's nuclear programme in exchange for sanctions relief.
They fear Iran has ambitions to build a nuclear bomb - something Iran denies, insisting it is merely exercising its right to peaceful nuclear power.
Negotiators are currently working towards a late-March deadline for an outline agreement with Iran, which would be followed by a detailed deal by the end of June.
But Mr Netanyahu said: "We've been told that no deal is better than a bad deal. Well this is a bad deal, a very bad deal."
Mr Netanyahu's rivals in Israel's election face a dilemma as the applause from the speech in Washington begins to die down.
It's been hugely frustrating for them to watch him walking the world stage and worse still watching the speech on television.
They are the kind of television images you cannot buy in an election campaign - the Houses of Congress rising repeatedly in standing ovations.
But if they repeat their warnings that Mr Netanyahu is endangering Israeli security by alienating the White House they risk protracting a story which has played to the prime minister's advantage.
Many of them, like the opposition leader Yitzhak Herzog, feel they're just as tough on Iran as Mr Netanyahu - they just don't have access to the world stage as he does.
Mr Netanyahu's Likud party is already asking voters if they can imagine any other Israeli politician making such a speech.
The opposition want to start talking instead about the cost of living and the other issues where they feel they have an advantage - they just hope images of Mr Netanyahu basking in a warm bath of applause fade before polling day in two weeks' time.