Clinton has quickly joined with one of President Barack Obama's top talking points and called for a ban on guns for anyone placed on the "No Fly List." "I've gotta tell you if you are too dangerous to fly in America you are too dangerous to buy a gun in America," Clinton said during a stop in Sioux City, Iowa, Friday.
Sanders on Thursday night used a Facebook post to call on Congress to allow the federal government to research "causes and effects" of gun-related events. "We must authorize resources for the Centers for Disease Control and Prevention to study and research the causes and effects of gun violence in the United States of America," the Facebook post said.
But that position is a reminder that Sanders, as congressman, in 1996 voted against barring the CDC from studying gun violence. Asked by CNN if he now regrets that vote, Sanders' campaign declined to say yes or no.
"He can't remember one vote 19 years ago out of more than 10,000 he's cast," Sanders' spokesman Michael Briggs told CNN. "But if the question today is whether he thinks we should find out as much as possible about what causes gun violence, the answer is, 'Yes.'"
Gun control is an issue that appears to continually trip up Sanders with the liberal base. And the shootings in San Bernardino, which left 14 dead and 21 wounded, show that it's an issue that will only reappear throughout the race with each subsequent tragedy.
Sanders' long career is peppered with quotes more in line with the National Rifle Association, than either his current positions on guns or the mood of today's Democratic Party.
"Anyone who has any illusions that gun control will cause a significant dent in the very serious problem of crime is mistaken," Sanders told Vermont's The Rutland Herald in 1991, following a shooting in his home state, where a woman walked into Eveready Battery Company and shot her boss, who died, and three co-workers.
Clinton has used the issue as a wedge.
In the middle of the first Democratic debate
, as he was being pressed if he was tough enough on gun control, Sanders said, "What I can tell Secretary Clinton, is that all the shouting in the world is not going to do what I would hope all of us want, and that is keep guns out of the hands of people who should not have those guns and end this horrible violence that we are seeing."
That comment later drew one of Clinton's strongest one-liners, accusing Sanders of sexism.
"I'm not shouting. It's just that when women talk, some people think we're shouting," she said in an oft-repeated line through the end of October.
In November, Clinton later nailed Sanders in the second debate for his vote in favor of protecting gun manufacturers from lawsuits.
"I said I made a mistake on Iraq," Clinton said in the second debate. "And I would love to see Senator Sanders join with some of my colleagues -- in the Senate that I -- see in the audience, let's reverse the immunity."
It's part of a lengthy history of votes and comments that have left gun control advocates suspicious of Sanders.
Democratic contenders had long been leery of addressing gun control since 1994 electoral wipeout they faced in part after President Bill Clinton signed the Brady Bill establishing background checks for gun purchases and the federal assault weapon ban. But a string of mass shootings across the nation, punctuated by the Sandy Hook Elementary School massacre in December 2012, led President Barack Obama to become a vocal leader on the issue, shortly after his 2012 re-election.
Tad Devine, Sanders' chief strategist, said that opponents are cherry-picking Sanders' record and ignoring key moments when he stood with the gun control community, beginning with his support in 1988 for an assault weapons ban.
"I dispute this notion that somehow Bernie Sanders is terrible on the issue of gun safety," Devine said. "He has a long record of supporting common sense gun safety measures."
Devine also pointed to Sanders' vote in the Senate Thursday with other Democrats in favor new gun controls.
But that may be not be enough for the senator.
Paul Helmke, former president of the Brady Center to Prevent Gun Violence, said Friday that when Sanders declined to say he made a mistake in voting against lawsuit reforms that would have made it easier to sue gun manufacturers, it reinforced concerns many already had in the anti-gun violence community.
"That's what people are suspicious of," said Helmke, now the director of the Civic Leaders Center at Indiana University's School of Public and Environmental Affairs. "It's good to get the convert, but the fact that it took him so long to become a convert is still looked at somewhat suspiciously. I think the general reaction is 'We're glad he's on this side now, wish he'd been on this side earlier. Is this just a conversion of convenience type issue?'"