Conservative commentator Ben Shapiro warned Republicans against underestimating Sen. Bernie Sanders' appeal with voters, as polls show the self-described "democratic socialist" with a slight lead in Iowa ahead of Monday night's caucuses.

"The appeal of Bernie Sanders...I think Republicans are underestimating," Shapiro said on Monday's "The Ben Shapiro Show."

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"I actually watched a Bernie rally beginning to end and it really tells you...everything Bernie Sanders does is about building a movement," he added.

An Emerson College poll released Sunday night found Sanders, I-Vt., with 28 percent support in Iowa, Joe Biden at 21 percent, Pete Buttigieg at 15 percent, Elizabeth Warren at 14 percent and Amy Klobuchar at 11 percent.

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According to Shapiro, Sanders' surge in the polls is in large part due to his messaging, which sets him apart from his Democratic opponents who focus their campaigns primarily on defeating President Trump in November.

"His whole shtick is 'We're not just going to defeat Donald Trump, who is the worst person to ever live, but also we're going to build a movement,'" Shapiro explained, "and this is why he has momentum right now."

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On Saturday, Sanders boldly predicted victory during a rally in Cedar Rapids, telling voters “we are going to win this election because we have the strongest grassroots movement in modern American history."

"Bernie's shtick is not the Bernie you actually see during the debates," Shapiro said. "During the debates, you have Bernie yelling all the time about 'unfairness...healthcare, and 'America is terrible.'"

"During his rallies, he speaks instead like some sort of UC Berkely 1968 professor ... he gets up and he says 'We're living in a country that has serious inequality and you and I together are going to make a movement.'"

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"It sounds much more reasonable," Shapiro concluded, "and it's designed like this. It's actually quite clever."