Democratic front-runner Sen. Bernie Sanders, I-Vt., was clearly "rattled' when members of the Charleston, S.C., presidential debate audience vocally pushed back on his praise of communist Cuba's education system, pollster Frank Luntz said Tuesday night.

That discord and Sanders' obvious irritation was the "secret recipe" for the other Democratic presidential candidates, Luntz said during an appearance on "The Ingraham Angle."

"Cuba made progress on education ... ," Sanders began in one instance -- before some in the audience shouted and booed.

DEMS PUMMEL SANDERS ON COMMUNISM AT SC DEBATE

"Really? Really?" a wide-eyed Sanders responded. "When dictatorships, whether it is the Chinese or the Cubans, do something good, you acknowledge that."

Sanders was "clearly" angry at that point, Luntz observed.

"In fact, this is the secret recipe for the other candidates," he said. "They put people, supporters, in the audience to challenge Sanders when he makes statements like that, and it rattles him. You've never seen that before."

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Luntz added that in some of the Super Tuesday states, those with open primaries, Republicans and independents can vote in the more-contested Democratic election.

In those races, Sanders will not be likely to get many crossover votes, Luntz said.

President Trump's nominal opposition in some of those races is former Massachusetts Gov. William Weld, so Republicans may choose to cast their vote in the other party's primary instead.

Luntz said former New York City Mayor Mike Bloomberg will surprise observers and win a handful of states on Super Tuesday, adding that Bloomberg had a much better debate showing than last week's Nevada bout.