Emotional pleas from former Rep. Gabrielle Giffords, D-Ariz., and her husband, Mark Kelly, on Tuesday tugged at senators both for and against a bipartisan proposal on background checks for gun purchases. It’s not clear whether they turned any votes, however, as negotiators were still scrambling to clear a 60-vote hurdle necessary for passage.

Vice President Joseph R. Biden Jr. said Democrats were two votes short of a filibuster-proof majority on an amendment that would end the gun show exemption and expand background checks to online sales.

Democratic aides tracking the bill said their magic number was a bit higher and said that members of both parties are still being lobbied. These same sources declined to give an exact whip count or list of names, given the delicate nature of the talks. An unofficial count by CQ Roll Call suggests that negotiators have 52 supporters, with eight senators still in play.

Kelly and Giffords appeared before Senate Democrats — including six still undecided on the measure — at the caucus’s weekly policy lunch. They later addressed a bipartisan crowd at the dedication of a room in the Capitol Visitor Center to slain Giffords staffer Gabriel Zimmerman, one of six people shot and killed in the 2011 Arizona mass shooting that rendered Giffords incapable of continuing her congressional service.

The poignant ceremony was marked by moments of awkwardness, as Kelly noted in his remarks the destructive power of high-capacity magazines that Congress cannot find enough votes to restrict.

Giffords’ friend, Sen. Jeff Flake, R-Ariz., was also at the ceremony, a day after he announced he could not support the bipartisan background check agreement.

Late Tuesday afternoon, Sen. Dean Heller of Nevada, who was one of three Republicans left on the board, announced he would not support the deal.

“I cannot support legislation that infringes upon the constitutional right to keep and bear arms,” he said of a compromise proposal authored by conservative Democratic Sen. Joe Manchin III, D-W.Va., and right-wing stalwart Sen. Patrick J. Toomey, R-Pa.

“I believe that this legislation could lead to the creation of a national gun registry and puts additional burdens on law-abiding citizens,” Heller said.

The language proposed by Manchin and Toomey in concert with two other senators explicitly prohibits the creation of a federal firearms registry “either directly or indirectly,” according to the bill text.

Sources tracking the talks said negotiators were trying to use changes to federal firearms licensing for remote, rural areas to woo senators from states such as Alaska and the Dakotas.

Moreover, the more changes made to the original language, and the weaker the law becomes, the more likely that Democrats on the left will drop off the bill.

House Action Moot?

With as much trouble as it’s having in the Senate, gun control legislation to expand background checks will face a steeper climb in the Republican-controlled House.

Perhaps the only way it could pass — a scenario that Democratic and Republican sponsors hinted at in a statement announcing their effort — is if Speaker John A. Boehner of Ohio brings the bill to the House floor in violation of the “Hastert rule,” which requires the support of a majority of Republicans.

“The American people are getting a vote in the Senate. They deserve one in the House,” Reps. Mike Thompson, D-Calif., and Peter T. King, R-N.Y., said in a joint statement announcing their bill, which is identical to the Manchin-Toomey Senate effort.

While not implausible — Boehner has been discarding the “majority-of-the-majority” precept at a much faster rate than his Republican predecessor — it is highly unlikely that the speaker will brook his conference on this issue, House GOP aides and other sources said.

And the issue may be moot, as it’s not clear whether Manchin and Toomey have the votes in the Senate.

“Our primary focus right now is the Senate,” said Andrew Arulanandam, director of public affairs for the National Rifle Association Institute for Legislative Action.

Thompson maintained an optimistic take that a vigorous lobbying campaign could end up persuading a big House majority to vote for the bill.

“I would be very, very surprised if you see mass numbers of people from either side of the aisle run away from this one,” Thompson said. “The only people that should be against this bill are criminals — people who know they can’t pass a background check.”

Thompson said he’s assuming “that the American people get their way.”

More realistically, given the views of Republican lawmakers on the subject and the power of the NRA, most Republicans won’t support the Manchin-Toomey bill, leaving a Hastert rule violation that depends on Democrats’ support as the most feasible route to passage.

To be sure, Boehner has shown a newfound willingness to violate the Hastert rule in the months after the 2012 elections.

J. Dennis Hastert, R-Ill., in eight years as speaker, violated his namesake rule 11 times, even though he maintained that it was an operating principle of his leadership. Boehner, in just over two years, has violated it five times, four instances coming in the past four months.

Even former Speaker Nancy Pelosi, D-Calif., went against the majority-of-the-majority practice at a slower rate than Boehner.

Boehner’s lapses have prompted backlash from some conservatives, including those mounting a failed coup plot that was partly fueled by the fiscal-cliff legislation, which passed with a minority of Republicans and a majority of Democrats.

John Gramlich contributed to this report.