LAS VEGAS — Joe Biden took aim at Bernie Sanders's votes on gun legislation earlier in his congressional career, calling them "immoral," in some of the harshest rhetoric the former vice president has used on the campaign trail against 2020 Democratic rivals.

"It's just flat-out immoral, it's just flat-out immoral," Biden said Thursday at a closed-door meeting with members of the anti-gun group Moms Demand Action. "You know, committed Republicans and some Democrats, like Bernie Sanders voted five times, five times against background checks and waiting periods notwithstanding the fact that millions of people were denied having access to weapons of destruction because they didn't qualify."

Biden spoke about legislation aimed at partially protecting gun companies from being held liable in civil court, which Sanders voted for in 2003 and 2005 when he represented Vermont's lone House district. Biden equated voting against those bills to shielding tobacco companies and drug manufacturers.

Biden also noted that in 1991 and 1993, Sanders voted against a mandatory waiting period for gun purchases. In 1993, he also opposed the Brady Bill, which was enacted by Congress and President Bill Clinton, requiring waiting periods and background checks. He also twice voted against an assault weapons ban, once in 1994 and again in 2013.

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Sanders, at the time, said he was representing the majority of rural residents in his state who opposed gun control. He served in the House from 1991 until 2007, when he joined the Senate. Since moving across the Capitol, he has largely favored gun control measures.

“I do think he’s changed his views, and I’m happy for that,” Biden said Thursday.

Those close to the Biden campaign have told the Washington Examiner in recent weeks that they believed Sanders's record on guns would provide an effective opening for attacks, particularly as Biden begins cratering in polls following two poor finishes in Iowa and New Hampshire.

Earlier this month, the Washington Examiner reported that Sanders ran to the right of his Republican opponent on guns in his 1990 congressional race.

That year, Sanders challenged GOP Rep. Peter Smith for Vermont's lone House seat and contended during the race that he was out of touch on multiple issues, including gun control.

Sanders attacked Smith for supporting a ban against assault-style weapons, saying he was "turning [his] back on the 30% who believe in no gun control."

"In the last year, polling in Vermont has shown that 65 to 70% support some kind of gun control, so it is not a great act of courage to adopt a position supported by 70% of the population," Sanders said at the time. "Clearly, this was a very pointed political strategy, in which you turn your back on the 30% who believe in no gun control, and then turn to the 70% who support your new position. I think people will see through that."

[ Also read: The Republicans' 2020 plan to turn every Democrat into a 'Bernie Bro']