President Barack Obama has secured congressional support for his landmark nuclear deal with Iran.

On Thursday morning (AEST), US Democratic Senator Barbara Mikulski announced she would be the crucial 34th politician to support the already signed agreement, giving Democrats the necessary votes to prevent Republican sabotage.

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Senator Mikulski’s support will enable Democrats to uphold President Obama’s veto of a Republican resolution of disapproval. And it spelled failure for opponents of the international agreement who sought to foil it by turning Congress against it.

Leading that effort were Israel and its allies in the US, who failed to get traction after spending millions of dollars trying.

The agreement signed by Iran, the US and five other world powers limits Iran’s nuclear program in exchange for hundreds of billions of dollars in relief from international sanctions.

Republicans and Israeli officials contend that concessions made to Iran could enable the country to wreak havoc throughout the Middle East.

Prime Minister Benjamin Netanyahu, who had personally lobbied US MPs to block the nuclear pact, will continue fighting the agreement, an Israeli official said.

Marshall Wittmann, spokesman for the pro-Israel lobby AIPAC, said his group also would continue rallying opposition to the nuclear agreement.

“No deal is perfect, especially one negotiated with the Iranian regime,” Mikulski said in a statement.

She called the accord “the best option available to block Iran from having a nuclear bomb”.

“For these reasons, I will vote in favour of this deal,” the senator said.

With opposition to the agreement failing to take hold on the Democratic side, supporters may even be able to muster the 41 votes needed to block the disapproval resolution from passing in the first place, sparing Obama from having to use his veto pen.