But with a vote scheduled for Wednesday afternoon, he had not yet secured the support needed to advance the plan as of Tuesday night, said two Senate aides who spoke on condition of anonymity. Nine gun proposals are scheduled for votes Wednesday.
The effort received a poignant lobbying boost from former U.S. Rep. Gabrielle Giffords (D., Ariz.), who was shot in 2011 and now leads gun-safety efforts. Giffords and her husband, Mark Kelly, met Tuesday with Toomey and Manchin, then went to the Capitol to press Senate Democrats for support.
Manchin and Toomey's proposal would expand existing background checks on gun buyers to cover firearms sold at gun shows and via the Internet.
Razor-thin passage could hinge on Lautenberg, who has been ill and has not come to Washington in more than a month - his last vote was Feb. 28 - but may return for the crucial vote.
"Sen. Lautenberg is feeling better and hopes to be in Washington for gun votes this week," spokeswoman Caley Gray wrote in an e-mail.
Lautenberg, 89, has long fought for tougher gun laws, and the Toomey-Manchin proposal would be the most significant new measure in 20 years. But the senator has been suffering from leg pain and muscle fatigue, and doctors have restricted his travel, according to aides.
The pain has forced him to use a wheelchair and is related to chemotherapy treatments for cancer several years ago, the New York Times reported. A New Jersey Democrat confirmed that report and said Lautenberg was loath to be seen in a weakened state.
Toomey spent Tuesday making phone calls, and he spoke at Senate Republicans' weekly lunch.
"I'm encouraging [colleagues] to read the bill," Toomey said before the GOP's afternoon meeting. "If they read the bill, and they look at what it actually does, I think they're much more likely to come on and support."
Later in the day, however, Sen. Dean Heller (R., Nev.) announced he would oppose the plan in part because he feared it would lead to creation of a "national gun registry" - a result specifically barred by language within the bill.
Many senators "are just looking for a reason to get to 'no,' " Giffords' husband said Tuesday at a breakfast hosted by the Christian Science Monitor.
The proposal faces an even tougher road in the House, though suburban Republicans formally introduced a companion bill Monday night. U.S. Reps. Mike Fitzpatrick, of Bucks County and Pat Meehan of Delaware County, are cosponsors on the bipartisan House version.
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