US President Barack Obama on Wednesday earned sufficient congressional backing to ensure the Iran nuclear deal does not get blocked, when a veteran Democrat became the crucial 34th senator to back the controversial accord.

Most US lawmakers are opposed to the deal, which would ease punishing economic sanctions on Tehran while preventing it from advancing its nuclear program. Many Republicans warn that the Islamic republic will seek to cheat its way to an atomic bomb.

But with Senate Democrat Barbara Mikulski announcing her support Wednesday, the deal now has 34 backers in the 100-member Senate -- the magic number needed to uphold a certain Obama veto should Congress pass a resolution that disapproves of the deal.

Overcoming a veto requires a two-thirds vote in the Senate and House of Representatives.

While the deal is not perfect, Mikulski said, "I have concluded that this is the best option available to block Iran from having a nuclear bomb."

Some Republicans have urged walking away from the deal and renegotiating tougher terms with Tehran, but US negotiators have warned that such a move could see the fragile international coalition that secured the agreement fall apart.

"At best, sanctions would be porous, or limited to unilateral sanctions by the US," said Mikulski, the longest-serving female senator in history.

The White House said it was "encouraged" that more than a third of Senators were in line.

"This strong support is a validation of the outreach that the president and his team have organized," White House spokesman Josh Earnest told reporters.

Republicans are unified in their opposition, warning that the deal leaves a civilian nuclear program intact, and that sanctions will provide Iran with a windfall of up to $150 billion, which they say could be funnelled toward terrorist operations.

Almost all 17 Republican presidential hopefuls are openly opposed.

"I believe that the American people are going to look back on this and say this was the single worst thing this president's ever done," candidate and New Jersey Governor Chris Christie told Fox News after Mikulski's announcement.

"Every death that Iran causes is now on Barack Obama's head."

- 'Zero tolerance' -

Secretary of State John Kerry, delivering a speech in Philadelphia on the Iran deal, said the Islamic republic will be required to live up to the agreement in full before it starts benefitting from sanctions relief.

"Without this agreement, Iran's so-called breakout time (to produce enough fuel for a nuclear weapon) was about two months. With this agreement it will increase by a factor of six, to at least a year, and will remain at that level for a decade or more," Kerry said.

He also reiterated arguments he has been making in the two months since the deal was signed, insisting it is not based on trusting Tehran but on its ability to police its activity.

"The United States and the international community will be monitoring Iran non-stop and you can bet that, if we see something, we will do something," he said.

"The standard we will apply can be summed up in two words: 'Zero tolerance.'"

Kerry made no mention of reaching the 34-vote Senate threshold in his speech, seeking to avoid the image of a victory lap as he aims to broaden support for the deal.

But he told CNN that the administration "will continue to try to persuade people up until the last moment."

Having been assured that the deal will survive, the White House is now eyeing another key threshold. If it gains backing from seven of the remaining undecided Senate Democrats to boost its numbers to 41, it could prevent Republicans from reaching the 60 members needed to force a vote on the resolution of disapproval.

That would save Obama the embarrassing step of cobbling together a minority coalition to sustain his veto preserving his landmark agreement, a process that would sow doubts among a skeptical American public and Washington's international partners, which are keen to see strong US commitment to the accord.

Two Democratic senators stand opposed to the deal, as do a handful of House Democrats.

But House Democratic leader Nancy Pelosi insisted Wednesday she would have sufficient votes supporting Obama.

"I am confident we will sustain the president's veto in both houses of Congress," Pelosi said.

Congress will vote later this month on the accord, which was reached in July between Tehran and six world powers: Britain, China, France, Germany, Russia and the United States.

Among Democrats in support, many have expressed deep concern about Iran's adherence to the deal, and stressed it is the best of bad options.

"The alternative, to me, is a scenario of uncertainty and isolation," Senator Chris Coons said Tuesday as he announced his support.