Israeli Prime Minister Benjamin Netanyahu has launched a US television news blitz to denounce the White House's preliminary deal to rein in Iran's nuclear programme.
Dismissing the pact as a deeply flawed framework, Mr Netanyahu pressed American lawmakers not to give Tehran "a free path to the bomb".
He said he had spoken with nearly two thirds of US House of Representatives' members and a similar number in the Senate, both Democrats and Republicans, about the issue.
The agreement was announced on Thursday in Lausanne, Switzerland , after years of negotiations between the US, Britain, France, Russia, China and Germany on one side and Iran on the other.
The negotiations, which have a 30 June deadline for final agreement, aim to significantly pare back Iran's nuclear technology while easing international sanctions on Tehran.
If enacted, the deal would strip down Iran's programme, which it insists is peaceful, for a decade and curtail other parts of it for another five years.
But Mr Netanyahu, who has said Iran's nuclear efforts threaten Israel's very existence, believes the deal leaves too much of Tehran's programme intact.
He told CNN's State of the Union: "This is a world issue because everyone is going to be threatened by the pre-eminent terrorist state of our time, keeping the infrastructure to produce not one nuclear bomb but many, many nuclear bombs down the line."
Appearing on NBC's Meet the Press, Mr Netanyahu said: "I'm not trying to kill any deal.
"I'm trying to kill a bad deal."
Meanwhile, the chairman of the Senate Foreign Relations Committee said Congressional scrutiny of the agreement would be vital.
"It's very important that Congress is in the middle of this, understanding, teasing out, asking those important questions," Senator Bob Corker told Fox News Sunday.
Republicans, who control both chambers of Congress, and some Democrats are preparing legislation which would entail a vote in Congress on any Iran deal.
Mr Obama has said he would veto legislation demanding an up-or-down vote on the pact.
It is unclear whether opponents of the deal would be able to muster the votes needed to override such a veto.
Relations between Israel and the US were badly damaged when Mr Netanyahu last month addressed Congress to attack the nuclear talks, in a trip organised without consulting the White House in advance.