But a critical mass of lawmakers from both parties remains opposed to expanded background checks, and bans on assault weapons and high-capacity magazines have already been declared politically unfeasible.
A few months ago, the background-check bill seemed like the starting point for new gun laws. Instead of a slam dunk, though, Democrats are now lining up for a last-second buzzer beater to try to salvage a narrow victory.
The difference between the progress the left has made on same-sex marriage and immigration and its struggles on guns comes down to intensity.
Both parties saw President Obama embrace same-sex marriage and use that issue to run up the score among young voters on his way to reelection.
Similarly, Hispanics - a key demographic group - hammered Republicans in the last election. The GOP has taken notice.
Those blocs acted in the polls that matter most - the ones that are open on Election Day.
Moreover, with young people strongly backing same-sex marriage and softer immigration laws, politicians risk obsolescence if they don't adapt.
But there isn't the same generational divide on guns, and it's hard to think of an instance of gun-control advocates similarly punishing lawmakers who have stymied their work.
"There's no sense that the future is inherently a gun-control future," said Michael Dimock, director of the Pew Research Center for the People & the Press.
And when it comes to voting patterns, there has long been an "intensity gap" between supporters of gun rights and gun control, Dimock said.
Nearly one in four voters who call gun rights a top priority has donated money to the cause, against 5 percent of those who prioritize gun control, according to a Pew poll released in January.
In the last election cycle, gun- rights organizations and advocates gave $4.2 million directly to federal candidates, parties, and outside spending groups, according to the nonpartisan Center for Responsive Politics. And that doesn't count more than $25 million the National Rifle Association spent independently.
Groups and individuals backing gay and lesbian rights gave $5.5 million.
For gun-control groups, the comparable giving was $5,036.
Pollster Scott Clement, writing on the Washington Post's website, pointed to an illuminating analysis by political scientist Jonathan Bernstein.
"Those people who have been pushing for marriage equality? They were calling for change. And marching for it, demanding it, donating money to get it, running for office to achieve it and supporting candidates who would vote for it," Bernstein wrote on his blog. "In many cases, they based their entire political identity around it."
Public opinion alone is passive, and "in politics," he wrote, "passive doesn't get results."
No one who has watched the debate on guns would call gun-rights advocates passive. Many gun owners treat the Second Amendment as the determining factor in how they vote, and they are willing to rise up against politicians who cross them.
"To somebody who is a gun owner or feels strongly about gun rights, it is an extremely tangible issue," Dimock said. He gave an example: An owner might worry that background checks would get in the way of passing on a gun to a relative.
Gun-control advocates haven't built that kind of "single issue" passion. Their supporters often prioritize other concerns.
"The other side has always been more vocal," acknowledged Shira Goodman, executive director of CeaseFirePA.
She insisted that change is building. Goodman described gatherings at synagogues and churches since Newtown, and people organizing in their basements. "We think we're going to get 100 people" at an event, and instead, she said, "we get 200."
Her group is planning a Tuesday rally outside the Philadelphia office of U.S. Sen. Pat Toomey (R., Pa.), hoping to push him to support a background-check bill and provide a critical seal of approval from a Republican with a strong pro-gun record.
Others are working to build a counterweight to the NRA. New York Mayor Michael Bloomberg has vowed to put some of his fortune behind efforts to elevate gun control to the kind of issue that can make or break campaigns. Former U.S. Rep. Gabrielle Giffords, gravely wounded during a 2011 rampage in Tucson, Ariz., is leading another advocacy group.
Obama last week urged voters to ramp up pressure on lawmakers, and he is planning another speech on guns Monday in Hartford, Conn.
Converting opinion into action is hard. For many, the political system is a distant behemoth, and many issues compete for the public's attention. But sometimes small steps matter. Lawmakers track how many phone calls they get on a given issue, and where the opinions fall.
"We need to be vocal and loud and make some noise," Goodman said.
The main thing officeholders notice, though, is wins and losses. For gun-control advocates to advance reforms, they have to help deliver victory for their friends and defeat for their foes.
"You want to be a leader, you want to be elected, you have to be on the right side of the issue," Goodman said. "That's the message we have to carry out."
If political careers start ending over opposition to gun control, you can bet change will follow.
Contact Jonathan Tamari
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follow on Twitter @JonathanTamari. Read his blog 'Capitol Inq' at www.philly.com/CapitolInq.