Nancy Pelosi, the Democratic leader in the House of Representatives, on Monday wrote to colleagues backing the nuclear agreement with Iran, Reuters reported.
Pelosi’s letter came as congressional Republicans railed against a UN vote on the deal.
"As you may be aware, I believe that this agreement is a major accomplishment. I am pleased that the response thus far from House Democrats has been so positive," she said in the letter released by her office and quoted by Reuters.
Pelosi noted in the letter that 100 former U.S. ambassadors, both Democrats and Republicans, wrote last week supporting the pact and that dozens of national security experts from both parties issued a statement backing it on Monday.
Pelosi's backing is considered essential for the landmark deal to survive congressional review.
The UN Security Council backed Iran's nuclear agreement with world powers on Monday, but the Islamic Republic's Revolutionary Guards attacked the resolution, underlining powerful opposition to the deal.
Republican lawmakers in Washington expressed anger at the UN vote, meanwhile. Several issued statements calling it an "affront to the American people" because it took place before the end of the congressional review period.
Senator Marco Rubio, a candidate for the 2016 Republican presidential nomination, called the day of the vote "Obama's Capitulation Monday."
Congress received the Iran nuclear agreement on Sunday and have 60 days starting Monday to decide whether to approve or reject it.
Despite agreeing to allow lawmakers to review the Iran deal, President Barack Obama has threatened to veto any legislation passed by Congress blocking it.
A law signed by Obama in May gives Congress the power to review and potentially reject a nuclear deal with Iran, but in spite of his willingness to allow Congress to debate the agreement, Obama has asserted it would be "irresponsible" to object to the deal and maintained he would veto any attempts to torpedo it.