Barack Obama says abandoning the nuclear agreement would open up the prospect of war.

Reuters/Washington

US President Barack Obama's hopes of preserving the nuclear deal between Iran and world powers were dealt a setback on Thursday when Chuck Schumer, one of the top Democrats in the US Senate, said he would the oppose the agreement.

Schumer's opposition, announced in a lengthy statement, could pave the way for more of Obama's fellow Democrats to come out against the nuclear pact announced on July 1 between the US, five other world powers and Iran.

The New York senator is among the most influential Jewish lawmakers in the US. He was the first Senate Democrat to announce his opposition to the agreement.

Another influential Jewish lawmaker, US Representative Eliot Engel, the top Democrat on the House of Representatives Foreign Affairs Committee, also said on Thursday he would oppose the nuclear pact in a statement obtained by Reuters.

Israeli Prime Minister Benjamin Netanyahu has been pushing lawmakers to oppose the nuclear agreement, which he considers a threat to his country's survival. Some pro-Israel groups have also been spending millions of dollars on an advertising campaign to push members of Congress to vote no.

Lobbying effort

Obama has been engaged in his own lobbying effort, including a combative speech on Wednesday in which he said abandoning the agreement would open up the prospect of war.

Speaking at a news conference on a visit to the Vietnamese capital Hanoi, US Secretary of State John Kerry, who negotiated the deal on the US side, said he respected Schumer and Engel but added that "rejection is not a policy for the future."

"It does not offer any alterative and many people in arms control and others have actually pointed that out. While I completely respect everybody's individual right to make a choice, I obviously disagree with the choice made," he said.

The US Congress has until September 17 to consider a resolution of disapproval of the Iran deal, which would eliminate Obama's ability to waive all sanctions on Iran imposed by the US Congress, a key component of the agreement.

Lawmakers will begin debating whether to reject the deal when they return from their August recess on September 8.

Schumer insisted he was not influenced by party or politics and had not been pressured.

"Advocates on both sides have strong cases for their point of view that cannot simply be dismissed. This has made evaluating the agreement a difficult and deliberate endeavor, and after deep study, careful thought and considerable soul-searching, I have decided I must oppose the agreement and will vote yes on a motion of disapproval," he said.

Obama has promised a veto if the resolution is passed by the House and Senate.

Republicans would need at least 13 Democrats in the Senate and 44 in the House to vote against Obama to muster the two-thirds majorities in both chambers needed to override a veto. So, while Thursday's announcements are a blow to the president, opponents of the deal still face an uphill battle to enact a disapproval resolution.

Other Democrats in favour

Several Democrats in both the House and Senate have already come out in favour of the nuclear deal, including Nancy Pelosi, the House Democratic leader. Schumer's colleague from New York, US Senator Kirsten Gillibrand, announced her support on Thursday.

A handful of House Democrats in addition to Engel have said they oppose the deal, including Representative Steve Israel, a member of the chamber's Democratic leadership.

Schumer said lawmakers would come to their own conclusions but he would try to persuade other senators to vote against the Iran deal. Schumer is currently the number three Democrat in the Senate and is in line to succeed Harry Reid as the party's leader in the chamber when Reid retires in early 2017.

A congressional aide said Engel would vote for a resolution of disapproval and also vote to override an Obama veto if the resolution passed Congress. However, Engel did not say he would lobby against the deal among other lawmakers.

The White House had no immediate comment on Schumer's announcement, which was distributed by the Senate Republican leadership after it was released by his office.