WASHINGTON: US President Barack Obama on Thursday told groups that support the Iran nuclear deal to make their voices heard to Congress to counter the millions of dollars in lobbying by those who want to derail the agreement.

"Right now the opponents of this deal have been flooding Congressional offices," Obama said on a call with groups including the Washington-based think tank Center for American Progress.

Groups who opposed the deal, such as American Israel Public Affairs Committee known as AIPAC, have spent $20 million in TV ads to put pressure on members of Congress, Obama said.

"They start getting squishy because they're feeling the political heat," Obama said of members of Congress he has met with in recent weeks.

Obama did not thank the groups for any of their support so far, but rather pressured them to step up their efforts.

He drew comparisons to the lead up to the Iraq war, noting that groups who opposed it were not vocal until it was too late.

“In the absence of your voices, you are going to see the same array of voices that got us into the Iraq war, leading to a situation in which we forgo a historic opportunity and we are back on the path of potential military conflict,” Obama said.

If upheld, the deal is certain to shape Obama's legacy as he prepares to leave office. He said he has never been more certain of a policy decision.

Congress is currently reviewing the deal that the United States and other world powers negotiated with Iran to limit its nuclear capabilities in exchange for a relief of sanctions.

Opponents of the deal question whether it goes far enough to ensure Iran does not develop a nuclear weapon.

Congress has until Sept. 17 to approve or reject the agreement. Obama has said he would veto any legislation that undermines the deal, but Congress could override his veto with enough votes.

Four Democratic representatives voiced their support for the deal on Thursday, including Senate candidate Chris Van Hollen and Dan Kildee, who represents the district of Amir Hekmati, who is being held in Iran.

The White House said its supporters so far have contributed to 70,704 emails and 63,862 calls to members of Congress, urging them to not reject the deal.

The White House did not provide a list of all the groups on the call.