It also prompted a storm of competing reaction.
From the left, gun-control advocates such as President Obama, New York Mayor Michael Bloomberg and Philadelphia Mayor Nutter praised Toomey's leadership, after long seeing him as one of many Republican roadblocks.
From the right, Toomey's erstwhile allies lashed him.
The lobbying arm for the Heritage Foundation, the conservative think-tank led by Toomey friend and former Sen. Jim DeMint, put out a statement saying, "We expect more from Pat Toomey."
The deal - crafted by senators who have each received strong NRA ratings - added new momentum to the push to strengthen gun laws in the aftermath of December's Newtown school shooting.
While expanding the reach of background checks, the plan would exempt "personal transfers" - those not advertised online or in print. That would allow relatives, neighbors or members of the same church, for example, to give or sell guns to one another.
In a sign of the plan's potential to win over middle-ground lawmakers, two Philadelphia-area House Republicans, Pat Meehan and Mike Fitzpatrick, quickly endorsed it, with Fitzpatrick vowing to sponsor a House version. A third GOP congressman, Charlie Dent of Allentown, praised it. Support from moderate Republicans will be critical to getting any gun bill through the GOP-controlled House.
Even in the Democrat-controlled Senate, the proposal faces a fate so uncertain that Toomey did not commit to voting for it. Amendments yet to come could poison the final version for either side.
A key procedural vote looms Thursday. It's not clear how quickly the Toomey-Manchin plan can reach the Senate floor.
Obama praised the bill, though it stops well short of the universal background checks he sought. "There are aspects of the agreement that I might prefer to be stronger," he said in a statement. "But the agreement does represent welcome and significant bipartisan progress."
On policy, Toomey may not have given much ground - he pointed out that background checks already exist for many gun sales. But symbolically, a Republican endorsement for a new gun law carried significant heft.
Toomey said background checks are no "cure-all" but can reduce the likelihood of dangerous people obtaining firearms. "Nothing in our amendment prevents the ownership of guns by any lawful person," he said at a news conference in the Capitol. "Criminals and the dangerously mentally ill shouldn't have guns. I don't know anyone who disagrees with that premise."
Many found grounds for fiery criticism.
"Pat Toomey betrayed us and our Constitution," said Kim Stolfer, president of western Pennsylvania-based Firearms Owners Against Crime.
Stolfer said he had called the senator's cell phone but gotten no response. He compared the bill to giving a spark plug to someone with a flat tire.
"It's long past due for government to stop focusing on law-abiding citizens and start focusing on the real criminal behavior," Stolfer said.
Though Toomey and Manchin had talked with the NRA while preparing the bill, the group condemned it Wednesday, saying, "The sad truth is that no background check would have prevented the tragedies in Newtown, Aurora or Tucson."
Speaking to concerns about an "erosion" of owners' rights, Toomey said, "It simply hasn't happened." He pointed out that existing background checks had stopped many gun sales.
Under the proposal, records of new background checks would be kept by federally licensed gun dealers, who already keep records of checks for sales at gun stores.
If background checks result in an approved gun sale, the government would be required to destroy all identifying records - a provision supporters say would prevent creation of a gun registry, as some owners fear.
The plan would also expand some owners' rights. For example, dealers could sell handguns across state lines, and new language would add protections for gun owners traveling across states.
Toomey noted in a later conference call with reporters that he remained "strongly opposed" to banning assault weapons or high-capacity magazines.
Still, gun control groups praised him, hoping his role would help the bill attract conservative Democrats and perhaps additional Republicans.
Bloomberg's group, Mayors Against Illegal Guns, pulled down its ad pressuring Toomey on background checks. "He stepped up," said Shira Goodman, executive director of CeaseFirePA.
Others were more muted in their support. "Is it enough? No. Is it progress? Yes," said a Twitter post from Sen. Bob Menendez (D., N.J.).
Toomey, a former banker, has focused most of his energy on taxes, spending, and budgets since joining the Senate in 2011, forsaking cultural battles. But he said that once he realized gun legislation was likely to reach the Senate floor, he wanted to help craft a bill that wouldn't damage gun owners' rights.
But having a bill with hope of its advancing is a major step for gun-control backers. Until now, proposals such as bans on assault weapons have floundered.
The deal means both Pennsylvania senators have now moved in favor of new gun laws since Newtown. In December, Democrat Bob Casey reversed his previous position and said he'd support bans on assault weapons and high-capacity magazines. He has also said he would support expanded background checks.
The shifts perhaps reflect a change in gun politics in Pennsylvania. Statewide polls, like those taken nationally, show overwhelming support for wider background checks.
"I don't think this is a change in any way of my conservative record," Toomey said. But at another point he conceded, "People are going to have a wide range of opinions."
Contact Jonathan Tamari at email@example.com, or follow on Twitter @JonathanTamari. Read his blog, "Capitol Inq," at www.philly.com/CapitolInq.