While the two offer few major ideological differences, the Clinton campaign has sought to cast Sanders as out of touch with the party on the issue in recent days, ratcheting up its attacks while polls show support for Sanders nearly matching hers in Iowa -- the state that opens the nominating process.
During debates and at town hall-style events, Clinton, the former secretary of State, has repeatedly castigated Sanders for voting in favor of a 2005 law that protects firearms manufacturers and dealers from being held liable when their products are used to commit crimes. Then-Sen. Clinton did not back the proposal.
In response, Sanders announced over the weekend that he is open to legislation being circulated in Congress that would reverse the law. Sanders has described the decade-old bill as "large and complicated" and that his support stemmed from a desire to protect small-business owners.
"I’m pleased that this legislation is being introduced,” Sanders said of the new measure championed by Sen.
Sanders cautioned that he had concerns about the legislation, which in a Republican-controlled Congress faces an uphill battle, and said he would propose an amendment that would require the secretary of Commerce to monitor the impact of the measure on "non-negligent" small gun-shop dealers in rural communities. Sanders has represented Vermont, a state with a strong hunting and gun-owning culture, in Congress for two and a half decades.
"I do want to make sure that this legislation does not negatively impact small gun stores in rural America that serve the hunting community," he said of the legislation.
The reaction on Sunday from Clinton, hours before Democrats gather in Charleston, S.C., for a final debate before nominating contests begin, could be seen in her repeated use of the term "flip-flop" in describing Sanders' newfound position.
"I am pleased that Sen. Sanders has flip-flopped on legal immunity for gun makers and sellers," Clinton said on CBS' "Face the Nation."
Clinton's criticism of Sanders is more about weakening Sanders' momentum than it is about the issue, said Dick Harpootlian, a former South Carolina Democratic party chairman.
"These incremental votes are inside baseball," said Harpootlian, who is remaining neutral in the race. "The fact is Sanders is making them nervous. He's generating enthusiasm with the base. You see it in young people, you see it in his crowds."
Clinton also took another step Sunday. She insisted that Sanders and other members of Congress support ending the so-called Charleston loophole on background checks. Under an amended version 1993 Brady bill that Sanders supported, the sale of a gun can move forward if the federal government fails to complete a background check within three days.
That loophole was how Dylann Roof, who gunned down nine parishioners at the historically black Emanuel AME Church in a racially motivated massacre in Charleston in June, was able to purchase his .45-caliber handgun despite facing a felony drug charge.
Clinton demanded that Sanders change his mind -- again.
"I'm calling on him to also flip-flop in the right direction and sign onto legislation to change the Charleston loophole," she said.
While Clinton holds a strong lead in national polls, recent surveys from early nominating states Iowa and New Hampshire show her and Sanders locked in a tight race. An average of several polls in Iowa shows Clinton with just a 4-percentage-point advantage while in New Hampshire, Sanders has 6-percentage-point lead.
Clinton must "stop that momentum," Harpootlian warned.
"There has to be some concern there that a repeat of 2008, when they underestimated Obama, could come back around," he said.