“What this campaign is about, and I’m seeing it every day, is an excitement and energy that does not exist and will not exist in the Clinton campaign,” Sanders told The Post's John Wagner.

The comment has particularly galled the Clinton campaign, which believes that reports of an enthusiasm gap between the campaigns are greatly exaggerated.

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And so, at campaign offices from Waukee to Ames, Sanders's quote now adorns whiteboards and poster boards, put there by Clinton staffers and volunteers hoping to prove him wrong.

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The theme has also become a common feature of increasingly high-pitched fundraising appeals to supporters.

"Our opponent [read: Bernie Sanders] just doesn't seem to think we have the fight and enthusiasm to take this across the finish line," Clinton said in one fundraising email this morning. "I disagree."

Earlier this week, top Clinton aide Huma Abedin wrote to supporters asking them to pitch in, as well. Her email specifically took issue with the Sanders quote.

"I am so tired of Hillary's team being dismissed and written off like this," Abedin said. "On the road with Hillary every day, I see countless women, men, girls, and boys of all colors and creeds who are inspired by Hillary, and excited to make her vision a reality."

The Iowa caucuses are particularly driven by the energy and enthusiasm of a candidate's voter base, considering that they are held every four years at night, on a weekday, in bitterly cold weather.

Clinton campaign staffers point out that although the attention is often focused on the huge attendance at Sanders's rallies — such as his 20,000-person day in Minnesota this week — their candidate is the one with the most loyal supporters. In recent polling, for example, 51 percent of Clinton supporters in Iowa say they "strongly" support her candidacy, compared with 46 percent of Sanders supporters.