PORTLAND — It’s either this agreement or no agreement.

That was the consensus Wednesday evening during a forum hosted by Maine Sen. Angus King, former Maine Sen. George Mitchell and former U.S. Ambassador Nicholas Burns on the proposed nuclear deal between the United States, Iran and a number of other nations.

With all three panelists in strong support of moving forward with the deal, the forum morphed into a prolonged justification of the deal, though there was some discussion about weaknesses.

“Those who oppose this deal tend to base their opinions almost entirely on this agreement versus a perfect agreement,” said Mitchell, who in addition to his time in the Senate has been involved in peace deal agreements in Ireland and the Middle East.

“It’s either this agreement or no agreement,” Mitchell said. “I think there is very, very little chance that negotiations could ever be resumed if this deal falls through.”

King, who along with the rest of Congress will vote on the issue in early September — through a resolution either in support or opposition to the deal — has voiced strong support.

“What I’ve done is tried to immerse myself in this,” he said. “I ultimately ended up with a long list of flaws in the agreement but then ended up trying to think of the alternatives. … I was afraid we’d end up with the worst of all worlds: a weaker sanctions regime against Iran and an Iran unfettered by this agreement.”

Under the deal, economic sanctions on Iran that have been in place for years would be lifted when Iran proves it has removed centrifuges and uranium stockpiles, among other requirements. Iran will be allowed to enrich uranium but only to a level well below what could be weaponized, and the country would allow international inspectors into the country for 25 years.

That sunset provision and others in the proposed deal have made the prospect of passage through Congress uncertain, especially in the Senate, where two prominent Democrats have broken ranks with Obama to oppose the deal.

“The weakness is that (those restrictions would be) only for 15 years,” said King at one point during the forum, which attracted a capacity crowd at the University of Southern Maine’s Talbot Hall and filled several overflow rooms.

King, Mitchell and Burns agree that the onerous economic sanctions Iran is living under are poised to unravel, mostly because China and Russia are likely to resume trade with the nation. Mitchell said Congress voting down a deal that has been agreed to by so many nations would harm the United States’ credibility and ability to control Iran’s pursuit of nuclear weapons in any way in the future.

“Without a doubt, it would affect our credibility in the world,” said Mitchell, adding that if the U.S. ever had to assemble a coalition for military action against Iran in the future, other countries would point to the failed deal and say, “we had a chance to not have a war.”

Burns said: “We’re trying diplomacy. That’s a far better way to resolve this.”

To that, there was energetic applause.

King addressed a core argument among opponents of the deal: that many of the provisions would end in 15 to 25 years. King said he wishes the deal could be permanent.

“We’ll have roughly the same options in 15 years that we have now, but we’ve bought 15 years,” he said.

It would take 60 votes in the Senate, where the most fervent opposition is coming from, to pass a resolution against the deal, and 67 votes to override a presidential veto of that resolution. House members — mostly Republicans — have lined up in opposition. Republican Rep. Bruce Poliquin of Maine’s 2nd Congressional District is one of them.

“As I have often said, I will not support any deal that leads to the likelihood of nuclear weapons for Iran — ever,” Poliquin said in a written statement Wednesday after an Associated Press report that Iranian inspectors might be allowed to self-police a nuclear site.

Republican U.S. Sen. Susan Collins of Maine is undecided, according to a spokeswoman, though she has voiced concern about lifting an arms embargo and some of the details of the inspection regime. A recent Los Angeles Times article speculated Collins will vote against the deal.

Democratic U.S. Rep. Chellie Pingree of Maine’s 1st Congressional District has voiced support for the deal.

“I hope my colleagues in Congress don’t let partisan politics stand in the way of approving what could be a historic deal to stop the spread of nuclear weapons,” Pingree said earlier this summer.

Carolyn Vaughan traveled to Portland from Hallowell for the forum.

“I really think the panelists were trying to inform us of their point of view,” said Vaughan. “From my point of view, it was good that we had concrete information from the insiders. … Hearing Sen. King lay out all the facts around what he believes, that was very important.”

Burns said that no matter how the situation progresses, one thing will remain the same.

“The Iranians are not 10 feet tall. We are,” he said of the United States. “We’re so much more powerful than they are. If this deal falls through, we still retain the upper hand.”