Harry Reid is the highest ranking Democrat in the Senate and he expressed his backing for the Iran nuclear deal in a press statement on Sunday: "I strongly support the historic agreement with Iran and will do everything in my power to ensure that it stands," Reid wrote.

The Senate and House of Representatives plan to vote by September 17 on a "resolution of disapproval" to the deal reached between Iran and the world group headed by the US with Germany, the EU and the other permanent members of the UN Security Council. The resoluton was brought by Republicans.

Reid added that the agreement was "the best path to stop Iran from obtaining a nuclear weapon."

"First, this is a good agreement on the merits, imposing the toughest inspections and verification regime in history, and a diplomatic solution is certainly less costly in American blood and treasure than any possible military option," Reid said.

"Second, if the Senate rejects this agreement, the international community will not support an attempt to secure another and they will not support the sanctions regime. Those are hard facts," he added.

With 27 lawmakers backing the deal, its Republican opponents will find it difficult to reach the numbers they need in the Senate for their resolution.

House Minority Leader Nancy Pelosi has said House Democratic supporters have the votes necessary to sustain Obama's veto despite unanimous Republican opposition to the agreement.

Republicans, and the Israeli government, have criticized the deal which they believe makes too many concessions to Iran.

jm/bw (Reuters, AP)