Israeli prime minister Benjamin Netanyahu has warned the US that an emerging nuclear agreement with Iran paves that country's "path to the bomb".
Following Mr Netanyahu's speech to congress in Washington, US president Barack Obama pushed back sternly, saying America would never sign such a deal and adding that Mr Netanyahu was offering no useful alternative.
In the US spotlight for a day, the Israeli leader showed no uncertainty, declaring in an emotionally-charged speech: "This is a bad deal. It is a very bad deal. We are better off without it."
The event, arranged by Republicans, has aggravated the Israeli leader's already-strained relations with Mr Obama and gambled with the long-standing bipartisan congressional support for Israel.
Two weeks ahead of voting in his own re-election back home, Mr Netanyahu took to the podium of the US House where presidents often make major addresses, contending that any nuclear deal with Iran could threaten his nation's survival.
In a tone of disbelief, he said that Iran's supreme leader, Ayatollah Khamenei, "tweets that Israel must be annihilated - he tweets."
Republicans loudly cheered Mr Netanyahu in the packed chamber, repeatedly standing up to applaud.
Democrats were more restrained, frustrated with the effort to undercut Mr Obama's negotiations. House minority leader Nancy Pelosi, a Democrat, did little to hide her unease and later issued a blistering statement criticising what she called Mr Netanyahu's condescension.
At the White House, the US president said there was value in the current economic sanctions against Iran and also in the negotiations in Switzerland aimed at restraining Iran's nuclear ambitions.
"Sanctions alone are not sufficient," Mr Obama said.
"If Iran does not have some sense that sanctions will be removed, it will not have an interest in avoiding the path that it's currently on."
The administration said there is no deal yet, but mr Netanyahu insists he is privy to what is being put forth.
He declared: "If the deal now being negotiated is accepted by Iran, that deal will not prevent Iran from developing nuclear weapons.
"It would all but guarantee that Iran gets those weapons - lots of them."
He acknowledged that any deal would likely include strict inspections, but he said "inspectors document violations; they don't stop them".
Mr Obama declined to meet with the leader of Israel, a key US ally, during this visit. Vice President Joe Biden was on a trip to Central America and so his seat as president of the Senate was filled by Republican Orrin Hatch of Utah, the Senate president pro tempore.
As Mr Netanyahu spoke, US secretary of state John Kerry was holding a three-hour negotiating session with Iranian foreign minister Mohammad Javad Zarif in the Swiss resort of Montreux in hopes of completing an international framework agreement later this month to curb Tehran's nuclear programme.
According to Mr Netanyahu, the deal on the table offers two major concessions: that Iran would be left with a vast nuclear infrastructure, and that restrictions on Iran's nuclear programme would be lifted in about a decade.
"It doesn't block Iran's path to the bomb," the Israeli leader thundered. "It paves Iran's path to the bomb."
He said the US and the other five nations in talks with Tehran should keep up the pressure with economic sanctions, because Tehran needs the deal most.
"Now, if Iran threatens to walk away from the table - and this often happens in a Persian bazaar - call their bluff. They'll be back, because they need the deal a lot more than you do."
More than four dozen House and Senate Democrats said in advance they would not attend the event, highly unusual given the historically close ties between the two allies.
Ms Pelosi issued a statement saying she was "near tears throughout the prime minister's speech - saddened by the insult to the intelligence of the United States".
Senate Republican majority leader Mitch McConnell said the Senate would debate next week on legislation that would allow a congressional vote on any deal reached with Iran. He said legislation for stiffer sanctions could well be considered.
Senator Robert Menendez, a key Democratic sponsor of that legislation, objected to Mr McConnell's plan to fast-track the bill, bypassing the committee process and sending the bill straight to the Senate floor for debate and a vote.
"I am more than disappointed. I'm outraged," Mr Menendez said.
Mr Netanyahu's speech reverberated in Israel, too.
Isaac Herzog, who is running against Mr Netanyahu, said: "The painful truth is that after the applause, Netanyahu remains alone and Israel remains isolated and the negotiations with Iran will continue without Israel. It won't change the (US) government's position and will only widen the divide with our great friend and our only strategic ally."
In Tehran, Iranian foreign ministry spokeswoman, Marzieh Afkham said Mr Netanyahu's speech was a "deceitful show" and part of a campaign by hardliners in Tel Aviv ahead of the election in Israel.