“He can win by getting one-third plus one of either house, so he’s still got a great likelihood of success,” he said of the president.

However, he added that the next US President could review the deal.

McConnell said Republicans would universally oppose the deal, and that the President was therefore naturally focusing his lobbying efforts on members of his own party.

“The campaign of the president to get it approved will be entirely among Democrats, probably Democrats in very safe Democratic seats whose only fear of re-election would probably be getting a primary,” he said.

Senator Schumer’s dissenting voice was a boost to the “No” campaign, he added, but that was unlikely to be enough to defeat Obama.

“I hope we can defeat it, but the procedure is obviously stacked in the president’s favor,” McConnell told reporters. “We’ll see.”

He also welcomed news that the only Republican Senator who had been considering voting in favor of the deal, Arizona Senator Jeff Flake, had issued a statement saying he would now vote against it.

“I was pleased that he finally reached the same conclusion the rest of us did, that it was not in America’s best interests to support it,” McConnell said.

He warned that the deal was dangerous, and urged fellow Republicans to lead a “respectful” debate detailing its many flaws.

“It leaves the Iranians with threshold nuclear capability,” he said. “It has no impact on all of their other activities. They are the biggest sponsor of terrorism in the world today. The inspections are flawed.”