Furrowing his brow and casting his eyes downward, Casey expressed regret that he had not reconsidered his views as starkly after earlier massacres at Virginia Tech and in Aurora, Colo.
"The power of the weapon, the number of bullets that hit each child, that was so, to me, just so chilling, it haunts me. It should haunt every public official," said Casey, who won a second term six weeks ago while touting his opposition to gun control.
In the days since the shooting that killed 20 children and eight adults, the debate around gun laws has shifted. Democrats have called for reinstating the assault-weapons ban and barring magazines that hold more than 10 rounds of ammunition.
"If those two bills come before the Senate, I'll vote for both," Casey said. He said his decision amounted to being "summoned by your conscience."
"Work that we do doesn't always bring you to that kind of deliberation or consideration. This has, for me, and I have to, and I think I should, vote that way on those two" measures, he said.
Casey spoke on the day President Obama also backed the two proposals and launched a task force to reduce gun violence.
Though many Democrats have pressed for new gun curbs since the Newtown shooting, the most significant developments in Congress have been changes of heart from lawmakers such as Casey, who were legislative roadblocks but who now may be part of a growing coalition supporting new regulations.
His comments follow statements from several other pro-gun Democrats, including Senate Majority Leader Harry Reid of Nevada and Sen. Joe Manchin of West Virginia, who this week signaled that they, too, would be open to new restrictions.
Casey, who said he still has "strong" support for the Second Amendment, has held staunch gun-rights positions throughout his Senate career. After 32 people were killed at Virginia Tech in 2007, Casey said, "No man-made law will ever eliminate cruelty and evil from every human heart," and argued against new gun laws.
He made similar comments in July after a rampage that killed 12 at a movie theater in Aurora, and he reiterated his opposition to new regulations in a debate shortly before Election Day.
On Wednesday, Casey was asked why he did not reevaluate his stand after those earlier killings.
"We're all open to that charge, and that's a fair question. That should weigh on us as well, any public official in my position," he said. The killing of children so young makes Newtown feel different, Casey said, but he noted that that view was unfair to families of those who died in other assaults.
Pausing for a few seconds as he searched for words, he said: "I wish that maybe I had spent more time thinking about those other tragedies in the way that I have on this."
In Pennsylvania, where there is strong support for gun rights, Casey could pay a political price for changing his stance before he even begins the new term voters gave him in November.
"There's no question I'll be open to criticism, and I understand that," Casey said. "I just believe that in light of what's happened, in light of measures we can take to lessen the chances that will happen [again], that these are two steps we can take."
Kim Stolfer, chairman of the Pennsylvania-based Firearms Owners Against Crime, reacted Wednesday night by calling Casey "a political opportunist seeking to curry favor with a public that knows nothing about the current level of laws."
"Firearms are the most regulated commodity in this nation," Stolfer, of the Pittsburgh area, said in an interview. "Our rights are not meant to be toyed with at a moment's whim, and we don't predicate freedoms on what others do."
Casey, often defined by his sober and cautious public image, said he typically looks at issues from an analytical perspective. He said this shooting, though, prompted a uniquely emotional reaction.
"This isn't a tax debate. This is different," he said.
"I've been around government and public policy a long time, and I can't think of another time when I had these same feelings," he said. "I don't really care if people criticize me for having emotions about this. It probably helped me think about it in a different way."
He described the impact of seeing a CNN story about Newtown teacher Victoria Soto, killed while trying to protect her pupils, and the grief of her heartbroken sister. The senator said he was transfixed watching as he washed dishes at home in Scranton on Sunday night.
"That hit me very, very hard," he said. "I started thinking about my daughters, the way they get along, the way they love each other." His four daughters range in age from 16 to 24.
Over the weekend, his wife of nearly 30 years, Terese, challenged him on the issue and urged him to support new gun laws, Casey said.
"She confronted me several times," he said. "She was very direct and persistent."
Casey said his mind was largely made up on Sunday and Monday, though he did not say so publicly until Wednesday.
Despite his shift, Casey said he believes hunting is a "family tradition" in Pennsylvania and that people's right to have firearms to protect themselves and their homes should be respected. He does not own a gun or hunt, but while growing up in Scranton, Casey, 52, said, guns were neither unusual nor controversial.
"There weren't a lot of negative connotations attached to it," he said.
So he always opposed gun-control laws, he said. He has received "A" and "B-plus" ratings from the National Rifle Association.
He could be open to charges of flip-flopping, but "context makes all the difference in a case like this," said political scientist Michael Federici of Mercyhurst College in Erie. Casey's move, he said, matches a changing mood among the public and lawmakers.
"There are many voters out there who have probably flipped themselves because this is different," Federici said, adding that there was a distinction between changing positions for political expediency and changing because of a dramatic development.
Casey also has time on his side. After defeating Republican Tom Smith by nearly 9 percentage points, he has six years before he faces voters again - "an eternity" in politics, Federici said.
Casey said he did not know what the potential fallout might be.
"Others can probably analyze that better than I can," he said, "but I know what I have to do on this."
Contact Jonathan Tamari at email@example.com or follow on Twitter @JonathanTamari. Read his blog, "CapitolInq," at www.philly.com/CapitolInq.