And midway through the town hall, Manchin announced he would not join a Democratic filibuster of the Republican-backed vote to disapprove of the deal, thus mooting whatever larger tactical significance his decision might have.
Yet the fate of the Iran deal remains very much a live issue in West Virginia, where Manchin says he remains studiously undecided just days before he will return to Washington to cast votes on it. And in a state where public sentiment is running against the deal, Manchin has been unusually public in airing his struggle to come to a decision.
"With the good Lord as my judge, I am undecided," he told the crowd gathered at the University of Charleston. "I was leaning towards it, but I am absolutely, truly undecided."
That prompted cheers from the crowd of several hundred, many wearing shirts declaring "We Need a Better Deal." The crowd had to be moved at the last minute from a packed room to an auditorium seating 900.
Visit Manchin's Web site and you can peruse a list of 29 meetings, hearings and conversations he has had with various officials ranging from presidential cabinet members to foreign ambassadors. He is the only undecided senator to have scheduled town hall meetings strictly to solicit opinions on Iran
People on both sides of the deal could listen to Manchin's comments on the town hall and conclude that he'd taken their side. He delivered a mixture of facts and arguments both for and against the deal, often challenging questioners on both sides with the opposite perspective.
Manchin's extended deliberations seem to have paid some political dividends, in that activists on both sides paid tribute Thursday to his careful consideration of the matter. Though Manchin is three years from his next election and his political profile as a former two-term governor is formidable, his status as the Senate's most conservative Democrat hasn't shielded him from his state's dismal opinion of President Obama and congressional Democratic leaders.
He acknowledged the political dimensions of his decision in a radio interview Tuesday: "The political decision for me, if I'm making it strictly about me and my politics, would be to vote against it, because I know the overwhelming support. I can't pull that trigger right now, because if I do, I'm pulling it for myself because I think it's good politics for me. But I'm not sure it's good policy for the government."
After the final deal was announced in July, Manchin indicated he was inclined to support the deal. In the past month, however, away from Washington, he has moved himself squarely back into the undecided column.
Rabbi Victor Urecki of Charleston's Congregation B'nai Jacob, one of the most influential West Virginia voices counseling Manchin against the deal, said his efforts have been "extraordinary."
"He has done exactly what we wanted," Urecki said, but added that he has no idea where Manchin will ultimately end up: "I think he's deeply conflicted."
"He knows this is a politically loaded issue," said the Rev. Jim Lewis, an Episcopal minister, former Marine and longtime antiwar activist in Charleston. "Joe is thinking politically, and good, because it's going to be discussed politically ... but he's doing a good job of keeping the policy and the politics together."
Throughout the forum, Manchin was careful not to enthusiastically embrace arguments on either side.
"If anyone tells you they are 100 percent for this deal or 100 percent against this deal, they haven't read it," he said. "They haven't studied it. The best I could ever could be is 60 percent for, 60 percent against, and it'll probably come down, it'll be 51 percent for or against when I make the final decision."
But careful consideration may get Manchin only so far. At Thursday's town hall, deal opponents outnumbered supporters by a roughly 3-to-1 margin and calls to oppose the deal won vigorous cheers.
"Every time that this country has drawn a line, they've stepped over it and they've kept going and going," said one attendee. "These people don't even love their children enough to care whether they live or die, so how can they possibly care about us?"
"Iran has been killing Americans since 1979, and I don't see anything where they're going to stop," said another.
"I believe if you bless Israel, then God will bless you," a pastor said. "And if you curse Israel then God will have license to bring a curse on you."
They were plenty of moments where Manchin appeared to embrace the skeptics -- in particular, his grave doubts about the resolve of American allies to crack down if Iran is found to have breached the deal.
"I believe we'll catch Iran cheating no matter what, and I believe we'll catch them in the next six months," he told the crowd. "If we catch them in six months, are we going to double down?"
But there were suggestions that Manchin is preparing to buck the wishes of the state and support the deal. He made a public pledge to support a preemptive resolution authorizing war with Iran if it ends up developing a nuclear weapon.
And in perhaps the most unmistakable hint that Manchin could end up in support of the deal, he repeatedly invoked that towering figure of West Virginia politics, the late Sen. Robert C. Byrd, and his 2002 vote against authorizing the Iraq War.
"When we declared war on Iraq, how many of you in this room, honestly, would say that when Senator Byrd voted against that war in Iraq, man, did I think he made a big mistake?" he said. "I gotta be honest with you, I was one of the one who thought Senator Byrd made a big mistake. But basically, maybe he had more information than I was privy to."
Manchin left the room, after taking questions nearly a half hour longer than scheduled, promising a decision early next week.
And he suggested that day would be coming none too soon. A Marshall Univerisity student stood up midway through the forum to lament the deeply partisan and sometimes personal nature of the debate.
"Even though I'm majorly opposed to the deal," he said. "I honestly find it rather ridiculous the political game that it's turned into."
"Oh my," said Manchin. "Welcome to my world."