Former Vice President Joe Biden's vote as a senator to authorize the 2003 invasion of Iraq has become an issue in the 2020 Democratic primary, with the campaign of rival Sen. Bernie Sanders hammering on the decision as a key contrast between the two front-runners.

"It is appalling that after 18 years Joe Biden still refuses to admit he was dead wrong on the Iraq War, the worst foreign policy blunder in modern American history," Sanders advisor Jeff Weaver said in a statement Saturday. "Unlike 23 of his Senate colleagues who got it right, Biden made explicitly clear that he was voting for war, and even after the war started, he boasted that he didn't regret it."

Weaver's statement came in response to Biden surrogate and former Secretary of State John Kerry's Friday claim that while Biden voted to authorize the Iraq War, it was not a vote "for" the war.

Kerry, at the time a senator from Massachusetts, also voted for the war.

The secretary doubled down on the comments Sunday.

John Kerry is a liar. He’s lying about Biden’s views on the Iraq War because he knows that what Bernie is saying — that Biden, as a key Dem Senate Committee Chairman, supported the invasion of Iraq — and you can see in the video Kerry knows he’s lying to protect Biden. Just watch https://t.co/LtWvK7ZpK7 — Glenn Greenwald (@ggreenwald) January 12, 2020

Biden's history of support for the invasion of and subsequent war on Iraq is a matter of public record, something that Sanders surrogates and supporters focused on over the weekend.

"Hundreds of thousands of people died in the Iraq War that Joe Biden pushed America into," tweeted Sanders speechwriter David Sirota. "Allowing Biden to now pretend he opposed that war is appalling."

Hundreds of thousands of people died in the Iraq War that @JoeBiden pushed America into. Allowing Biden to now pretend he opposed that war is appalling.



You’re not an “attack dog” when you say that. You’re simply refusing to let history be revised. #JoeVotedForTheWar https://t.co/5kWoJNnLsl — David Sirota (@davidsirota) January 12, 2020

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Sirota helped get #JoeVotedForTheWar trending on Twitter on Saturday and Sunday by joining other users in posting clips from archival footage of Biden expressing his support for the Iraq War long past the initial invasion. Biden claimed in September that he supported the war up until it actually began.

“President Bush did not lash out precipitously at Iraq after 9/11...At each pivotal moment, he has chosen a course of moderation and deliberation, and I believe he will continue to do so.” —Biden explaining why the Senate should authorize the Iraq invasion pic.twitter.com/m1XXIsLQeu — Walker Bragman (@WalkerBragman) January 12, 2020 BREAKING: Video emerges of @JoeBiden criticizing antiwar Dems, praising Bush for leading America into the Iraq War & promising he will support Bush's continuation of the war



"The president of the United States is a bold leader & he is popular...I & many others will support him" pic.twitter.com/Sx2zsdbSJV — David Sirota (@davidsirota) January 12, 2020 Here's Joe Biden in 2002 championing the Bush administrations WMD lies#JoeVotedForTheWar pic.twitter.com/frabNtYM6T — jordan (@JordanUhl) January 12, 2020 Tell us more about how you supported the war in Iraq:https://t.co/QgolmJn6nJ — Socialist States of America (@Viatche87587881) January 13, 2020

As the New York Times reported Sunday, Biden's position on the war is becoming an issue just weeks before Iowans hold the nation's first primary contest on February 2:

Now, three weeks before the Iowa caucuses—held in a state with a fierce antiwar streak—the issue threatens to be a campaign liability for Mr. Biden as he seeks to assure voters of his ability to handle a foreign crisis even as he works to distance himself from a war that has had enormous costs for his own family, and for the nation.

The vote, said Fordham law professor Zephyr Teachout, is important context for the primary election so voters can understand candidates' priorities.

"He is running for Commander in Chief," said Teachout. "It matters. Why is he denying his own role?"