Supporters of the international nuclear agreement with Iran moved within one vote of mustering enough support to protect the deal in the U.S. Congress on Tuesday, September 1, when two more Democratic senators said they would support the pact, Reuters reports.

Senators Bob Casey and Chris Coons, known as Iran hard-liners, both said they backed the agreement announced on July 14 between the United States, five other world powers and Tehran.

Altogether 31 Senate Democrats and two independents who vote with them now support the deal, a potential legacy foreign policy achievement for Democratic President Barack Obama.

Backers will need 34 votes in the Senate or 146 in the House of Representatives to sustain Obama's veto if a Republican-sponsored resolution of disapproval passes both chambers.

Both Casey and Coons said they had had serious questions about the agreement, but decided it was the best option for limiting Iran's nuclear program and preferable to the United States breaking from the other countries that signed the deal.

"I was never persuaded by arguments that we could ... renegotiate successfully and get a result," Casey told Reuters in a telephone interview.

"I will support this agreement because it puts us on a known path of limiting Iran's nuclear program for the next 15 years with the full support of the international community," Coons said in a speech at the University of Delaware.