A second US Senate Democrat spoke out Tuesday in opposition to President Barack Obama's nuclear deal with Iran, but momentum within the party nevertheless appeared to be favoring the landmark accord.

Despite Senator Robert Menendez joining a congressional chorus of disapproval of the pact, Senate Republican Majority Leader Mitch McConnell conceded that Obama likely had the votes to ensure its survival.

"I have looked into my own soul and my devotion to principle may once again lead me to an unpopular course, but if Iran is to acquire a nuclear bomb, it will not have my name on it," Menendez said in a speech at Seton Hall University in South Orange, New Jersey.

"We should make it absolutely clear that we want a deal, but we want the right deal -- and a deal that does nothing more than delay the inevitable isn't a deal we will make," added Menendez, a former chairman of the Senate Foreign Relations Committee who has often crossed party lines in efforts to toughen economic sanctions on Tehran.

"I know that, in many respects, it would be far easier to support this deal, as it would have been to vote for the war in Iraq at the time," he said.

"But I didn't choose the easier path then, and I'm not going to now."

New Jersey's Menendez is only the second Senate Democrat to oppose the accord reached between Iran and six world powers, after Senator Chuck Schumer of New York.

Congress, where the GOP is united in opposing the Iran deal, is expected to pass a resolution of disapproval in September.

Obama is certain to veto the resolution, and he would need just one third of members in each chamber to back him to ram the deal through.

McConnell said Monday he hoped Congress could override a veto, but conceded Obama had "a great likelihood of success," the Lexington Herald-Leader reported.

- 'Temporary limitations' -

By Tuesday, 21 of the Senate's 46 Democrats were backing the deal, according to a CQ Roll Call count, after Hawaii's Mazie Hirono became the latest senator to support Obama.

Secretary of State John Kerry, who has lobbied lawmakers hard in support of the agreement, "remains confident that this deal... will get the support of Congress," State Department spokesman John Kirby told reporters.

Menendez said the agreement does not provide for sufficient inspections of Iran's nuclear or military sites nor destroy any centrifuges capable of separating the explosive uranium 235 isotope from uranium ore.

Critically, it lifts punishing economic sanctions on Iran "in exchange for only temporary limitations on its nuclear program -- not a rolling back, not a dismantling," Menendez said.

And should Iran violate the pact and make a dash for the bomb, "our solace will be that we will have a year's notice instead of the present three months" of known breakout time, a warning period he described as nothing more than "a very expensive alarm system."

Reaction to Menendez's speech was swift, with progressive advocacy group CREDO Action blasting him as a warmonger.

"Menendez's support for war over diplomacy isn't surprising, but that doesn't make it any less dangerous or irresponsible," the group's campaign manager Zack Malitz said.

While GOP backing for the Iran deal stands at zero in Congress, some prominent Republicans have spoken out in support.

They include Brent Scowcroft -- a national security advisor to president George H.W. Bush -- and ex-senator Richard Lugar, who warned Washington would be abandoned by international partners should Congress reject the deal and toughen Iran sanctions.

"This is the best opportunity we have to delay and potentially stop an Iranian nuclear situation," Lugar told MSNBC.