Toomey, taking a bold step that defied many on the right and in his party, became the second Pennsylvania senator to speak up for new gun laws - a change that would have been unthinkable as recently as November.
Though he and Democratic Sen. Bob Casey arrived at their positions through markedly different routes, Toomey's shift - and to a lesser extent, Casey's - reflects how the gun debate has changed in the aftermath of Newtown.
Toomey's Republican endorsement, in particular, has provided a jolt of momentum in the push to close longstanding gaps in the background-check law.
To be sure, the appetite for new gun laws has already shown its limits. Calls for bans on assault weapons and high-capacity clips quickly fizzled. Even though public opinion overwhelmingly favors stronger background checks, political opposition remains so tough that the proposal faces an uncertain fate in the Senate and a steep climb in the House.
But new gun limits are strongly favored in the suburbs, and whatever his bill's fate, analysts say Toomey can look forward to reaping political rewards from a stance that not so long ago would have seemed like anathema for a gun-rights Republican.
The deal he made with Manchin has burnished his credentials as a lawmaker willing to find bipartisan compromise, an important attribute for a GOP incumbent who will need a strong showing among moderate suburban voters when he faces reelection in 2016.
"Pat believes in this, he believes it's the right thing," said former Gov. Ed Rendell, who consulted with Toomey by phone and e-mail in the weeks leading to Wednesday's announcement. "It's also a very wise political decision."
Though gun owners are an active political bloc, Rendell, a gun-control advocate, cited his statewide wins and that of fellow Democrat Kathleen Kane, who was elected attorney general in the fall while backing tougher gun laws.
A short time later, with the images of Newtown still fresh, Casey spoke out in favor of banning assault weapons and high-capacity gun magazines, reversing his previous position. Casey told of seeing pictures of the children on TV as he washed dishes in his Scranton home. He called the images haunting and said he thought of his four daughters.
Toomey seemed more analytical. With President Obama and other Democrats pushing for new laws, a gun bill was heading for a vote, regardless of any GOP cooperation. Toomey wanted a hand in crafting it and perhaps finding a plan that could both pass and protect Second Amendment rights, he said.
"It's not something that I sought, but it's something that I think is inevitable," he said Wednesday with Manchin at his side.
At first glance, they made an odd pair. Manchin, a burly West Virginian, once ran a campaign ad in which he used a rifle to blast a hole in an environmental bill. Toomey, a lean ex-trader, had dedicated his public career to railing against taxes, targeting spending and touting "pro-growth" economic policy.
But the senators' agreement was the product of a personal relationship built over several years, months of quiet consultation by Toomey and days of negotiating.
After he joined the Senate in late 2010, Manchin was one of the first senators to invite Toomey to meet. Their states share a border, and some interests. They had worked together on coal issues and in February shared a flight to Pittsburgh for an energy conference. On the way, they discussed some of their common ground.
"It was just kind of a natural alliance," Toomey said.
Separately, as the push for new gun laws gained traction in December and January, Toomey asked his staff to monitor the talks in Washington.
He met with gun-control groups, who sensed he might be willing to endorse stronger background checks.
Toomey, who once said his idea of gun control was "a steady aim" and who was endorsed by the National Rifle Association in 2010, spoke with Rendell and Mayor Nutter, as well as the NRA.
In recent weeks, Toomey and Manchin spoke, with talks gaining steam in early April. Obama and Toomey spoke by phone Tuesday, shortly before the deal was completed.
Though Democrats had lobbied for universal background checks, the compromise carved out exceptions for "personal transfers" between relatives, friends, and acquaintances - a nod to gun owners. Under the plan, background checks would expand to cover gun shows and Internet gun sales.
On Wednesday, as Obama was rolling out his budget, Toomey stood before a jammed press room talking about guns.
"The common ground rests on a simple proposition," he said, "and that is that criminals and the dangerously mentally ill shouldn't have guns."
Honest gun owners, he said, would not give up any rights. "Nothing in our amendment prevents the ownership of guns by any lawful person," he said. Asked about political ramifications, Toomey replied, "What matters to me is doing the right thing, and I think this is the right thing."
Gun-control groups didn't get all they wanted, but they hailed Manchin and Toomey. Former Rep. Gabrielle Giffords (D., Ariz.), shot and seriously wounded in 2011 and now the leader of a gun-safety group, recorded robocalls praising Toomey and Manchin.
Some gun groups called the deal a "betrayal" that punishes honest gun owners. Many of Toomey's fellow Republicans will likely oppose the deal, including their Senate leader, Mitch McConnell of Kentucky.
In Pennsylvania, though, polls show that 85 percent to 95 percent of the public favors expanded background checks, said Chris Borick, a Muhlenberg College political scientist.
The deal is "a no-brainer" in the Philadelphia suburbs, home to a rich vein of swing voters who can tilt a statewide election, Borick said. Republican U.S. Reps. Mike Fitzpatrick, of Bucks County, and Pat Meehan, of Delaware County, quickly endorsed Toomey's compromise.
In 2016, Toomey will have to contend with a presidential election, which in recent decades has seen Pennsylvania's electorate go increasingly Democratic.
But his deal with Manchin is the kind of move with inherent appeal for moderates, even those who might favor a popular Democrat - say, Hillary Rodham Clinton - atop the 2016 ballot.
Elected amid the tea party wave of 2010, Toomey may have just neutralized any Democratic attempt to cast him as a right-wing zealot - even while giving only modest ground on policy; the power of his deal is more rooted in the bipartisan symbolism on such a divisive issue.
"If he wants to make his case that he's not just a rigid ideologue," Borick said, "this will be a powerful example."
Conversely, if Toomey had opposed an idea favored by nine out of 10 voters, he could have been painted as out of step.
"He would have been shredded in the Philadelphia suburbs," said Rendell. "You can win statewide if you get clobbered in Philadelphia. You cannot win statewide if you get clobbered in Philadelphia and the Philadelphia suburbs and the Lehigh Valley."
Toomey, who opposes bans on assault weapons and high-capacity clips, will still have stronger Second Amendment credentials than anyone the Democrats will offer, Rendell said.
"The NRA as an organization might be slightly pissed with Pat," he said, "but they'll get over it."
Contact Jonathan Tamari at email@example.com or follow on Twitter @JonathanTamari. Read his blog, "Capitol Inq." at www.philly.com/CapitolInq.