"What was the most important vote cast in the modern history of America on foreign affairs? Yeah, it was Iraq," Sanders told The Des Moines Register's editorial board Thursday.

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"What does Hillary Clinton have to do to convince you that she has significant foreign policy judgment? She cast a vote for the war. I cast a vote against the war," Sanders added.

Sanders is hoping to gain ground on the former secretary of State on foreign policy, which has moved toward the forefront in the wake of recent terror attacks in Paris and San Bernardino, Calif.

Clinton has since called the vote a mistake and has leaned on her tenure as secretary of State in debates to offer herself as the more experienced candidate on foreign policy matters.

Sanders is trying to make up ground on Clinton before the first-in-the-nation Iowa caucuses on Feb. 1.

Clinton leads the Hawkeye State with 50 percent support, according to a RealClearPolitics polling average, with Sanders in second at 37.5 percent.

Sanders had drawn even with Clinton in the state in the fall, but a surge by the former secretary of State since then has put her comfortably ahead.