Democratic presidential candidate 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's campaign is knocking rival 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 for skipping a Senate vote to crack down on North Korea after he called the isolated country a key concern.

"It is unfortunate that yet again, Senator Sanders has shown a lack of interest in vital national security issues, failing to vote on sanctions against the country he said poses the greatest threat to the United States," Clinton campaign spokesman Jesse Ferguson said after the Wednesday vote.

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Sanders — one of four senators to miss Wednesday's vote — said that while he had to be "necessarily absent," the increased sanctions were "absolutely essential" to ending North Korea's nuclear program.

The vote on the sanctions legislation follows the Vermont senator suggesting during last week's debate that out of North Korea, Russia and Iran, he was most worried about North Korea.

"I worry very, very much about an isolated country," he said at the time. "North Korea is a very, very strange country because it is so isolated, and I do feel that a nation with nuclear weapons, they have got to be dealt with."

Wednesday's criticism from the Clinton campaign comes as the former secretary of State's allies have increasingly suggested Sanders is inexperienced on security-related policy as the two have battled in early-voting states.

Sanders has rebuffed that criticism, pointing to Clinton's vote for the Iraq War.