Republican presidential candidate Mike Huckabee has been criticised by Democrats for saying the nuclear accord signed between six world powers and Iran will "take the Israelis and march them to the door of the oven".
The former governor of Arkansas made the remarks, an apparent reference to the Holocaust, during an interview with Breitbart News.
"[US President Barack Obama's] foreign policy is the most feckless in American history. It is so naive that he would trust the Iranians," Huckabee said.
"By doing so, he will take the Israelis and march them to the door of the oven.
"This is the most idiotic thing, this Iran deal. It should be rejected by both Democrats and Republicans in Congress and by the American people. I read the whole deal. We gave away the whole store. It's got to be stopped."
Republicans have strongly opposed the 14 July accord struck with Iran, which limits Tehran's nuclear capabilities in exchange for the lifting of international oil and financial sanctions, saying far too many concessions have been made.
The US Congress has until 17 September to approve or reject the agreement, although Obama would be able to use his presidential veto to overrule the Republican-dominated Congress in the event of a No vote.
Huckabee's comments were roundly condemned by senior Democrats, with the head of the Democratic National Committee saying they had "no place in American politics".
"This rhetoric, while commonplace in today's Republican presidential primary, has no place in American politics. Cavalier analogies to the Holocaust are unacceptable," Debbie Wasserman Schultz said.
"Mike Huckabee must apologise to the Jewish community and to the American people for this grossly irresponsible statement."