US Secretary of State John Kerry has taken the unusual measure of delivering a video message to Congress in an attempt to stop the body from collapsing last weekend's nuclear deal with Iran.
The American people need an insurance policy to prevent a rerun of North Korea.
Just back from his diplomatic triumph in Europe, Kerry made the message for legislators as he urged that they not introduce new economic measures against Iran at a time when the US and fellow world powers are withdrawing some sanctions in exchange for the Iranians curtailing their nuclear programme.
Kerry asserted that now is the time to get to work on a final agreement that removes any suspicion that Iran is trying to produce nuclear weapons.
"We all know that if the agreement falls apart, Iran is going to quickly face even tougher sanctions," he said in the message.
Although Kerry was reaching out personally to key senators, Democrats and Republicans appeared determined to increase the pressure on Tehran.
Two key senators already are at work on legislation to reinstate the full force of sanctions and impose new ones if Iran doesn't make good on its pledge to roll back its nuclear programme.
"The American people need an insurance policy to prevent a rerun of North Korea," said Senator Mark Kirk, a Republican who is drafting a bill alongside Senator Bob Menendez, a Democrat. Critics of the accord reached in Geneva believe it could allow Iran to trick international monitors while it assembles an atomic weapons arsenal, similar to North Korea last decade.
Israel sees any letup on the economic pressure as a dangerous concession that allows Iran to move even closer to nuclear weapons capability.
The American Israel Public Affairs Committee has joined the call, saying new sanctions are needed "so that Iran will face immediate consequences should it renege on its commitments or refuse to negotiate an acceptable final agreement."
Menendez and Kirk hope to have their bill ready for other lawmakers to consider when the Senate returns on December 9 from its two-week recess, according to legislative aides
The measure would require the administration to certify every 30 days that Iran is adhering to the terms of the six-month interim agreement and that it has not been involved in any act of terrorism against the United States.
Without that certification, sanctions worth more than $1bn a month would be re-imposed and new sanctions would be added.
The new penalties would include bans on investing in Iran's engineering, mining and construction industries and a global boycott of Iranian oil by 2015.
Kerry, whose message sought to push back against what he called "misinformation," spoke privately by telephone with Menendez in an effort to sway him, officials said.