A vote to allow Amtrak passengers to carry guns in their checked luggage is the latest front in a fight over firearms between Democratic presidential candidates Hillary Clinton Hillary Diane Rodham ClintonHillicon Valley: FBI chief says Russia is trying to interfere in election to undermine Biden | Treasury Dept. sanctions Iranian government-backed hackers The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters FBI chief says Russia is trying to interfere in election to undermine Biden MORE and Bernie Sanders Bernie SandersMcConnell accuses Democrats of sowing division by 'downplaying progress' on election security The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters Why Democrats must confront extreme left wing incitement to violence MORE as polls tighten in their race for the nomination.

At Sunday night's debate, Clinton accused Sanders of voting "to let guns go onto the Amtrak," referencing a 2009 bill that allowed passengers to pack guns in their checked luggage for train trips.

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Sanders has countered that Clinton is being "very disingenuous" in attacking his votes on measures like the 2009 Amtrak gun bill. Clinton has for several weeks tried to put Sanders on the defensive over his record on gun control.

The 2009 bill tied Amtrak's federal funding to the company allowing passengers to carry guns in their checked bags. The measure was included in a funding bill for the Department of Transportation and the Department of Housing and Urban Development that President Obama signed into law.

Sanders was one of 68 senators who voted for the amendment containing the Amtrak gun provisions, joined by 27 Democrats and 40 Republicans.

The Independent Vermont senator has said Clinton is smearing his record on guns to score political points.

"I have a D-minus voting record from the NRA [National Rifle Association]," he said in Sunday's debate. "I was in 1988, there were three candidates running for Congress in the state of Vermont. I stood up to the gun lobby and came out and maintained the position that in this country, we should not be selling military-style assault weapons."

The Amtrak measure was one of several Clinton cited to argue Sanders isn't tough enough of gun control.

"He voted against the Brady Bill five times," Clinton said in the debate, referring to the Brady Handgun Violence Prevention Act.

"He voted for what we call the 'Charleston loophole,' " she continued. "He voted for immunity from gun-makers and sellers which the NRA said 'was the most important piece of gun legislation in 20 years.' He voted to let guns go onto the Amtrak, guns go into National Parks. He voted against doing research to figure out how we can save lives."

Amtrak said it began allowing passengers to carry guns in their checked luggage on Dec. 15, 2010. The company requires passengers to notify it that they intend to pack firearms or ammunition at least 24 hours before their train trips.

The company says in a post on its website that "passengers must travel on the same train that is transporting the checked firearms and/or ammunition.