The head of one of the most powerful progressive organizations in Iowa says Bernie Sanders’ momentum from rallies in the state is making the Iowa Democratic caucus impossible to predict.

“It’s been an incredible appeal what he has had,” Progress Iowa executive director Matt Sinovic said of Sanders, appearing on “Breitbart News Sunday” on Sirus/XM 125 with host Steve K. Bannon and Washington political editor Matt Boyle, which was broadcasting from Des Moines. Progress Iowa is a nonprofit organization that describes itself as “powered by a community of more than 30,000 progressives, focused on research, education, and advocacy.”

Sinovic described Sanders as “a movement candidate that has caught fire” competing against Clinton, who has a “broad, broad base of support” culled from a prior campaign.

“So you have these two forces that are going to a come to a head tomorrow night. That’s why it’s been so hard for people to predict,” Sinovic said.

“The Sanders campaign how now crossed the threshold of having more than fifty thousand people appear at his rallies over the course of this campaign in Iowa.”

How will higher voter turnout benefit either Sanders or Clinton?

“I think it probably helps Bernie, but it really depends on who gets their people out,” Sinovic said. “I think it’s less about the number of turnout versus who shows up. If it’s a lot more first-time caucus-goers, that’s more Bernie. The Clinton campaign has all these institutional endorsements, and if they turn their regular folks out a lot, and get them out, then it’s going to be more Clinton.”

“The conventional wisdom…it is anybody’s game…the Sanders camp is going to do particularly well in college towns and the Clinton campaign is going to do well in the more rural areas.”

“If something changes that [college] dynamic, you could be looking at a better night for Hillary than is expected, or if Sanders does better in some rural counties, that might show that it’s trending in his direction.”

“At a grassroots level, you get it that people are, especially from an economic perspective, they think that the system is rigged against them. And I think that has come through,” Sinovic said, noting that not all people who feel this way are necessarily in the Sanders camp.

“That economic angst is going to translate into a lot of the rest of the primary campaign.”

The Clinton-Sanders battle is reaching fever pitch with just hours to go until the caucus.

.@BernieSanders doubts he can match Pres. Obama's 2008 Iowa turnout: "But I think it will be high enough" to win.https://t.co/fiBCFlUOCy — ABC News Politics (@ABCPolitics) January 26, 2016

Sanders campaign manager Jeff Weaver accused the Clinton campaign of “infiltrating the caucuses with out-of-state paid staffers,” including a precinct captain in Hawarden, Iowa who is not a resident of the state. The rules do not prohibit using out-of-state precinct captains.

“Coming from Sioux City into the Hawarden caucus and trying to influence the voters in the Hawarden caucus, I don’t know what the rules are, but it certainly is irregular,” a Sanders supporter told Yahoo News.

Sanders held a massive rally with the band Vampire Weekend in Iowa City, home of the University of Iowa, on Saturday night. Bill Clinton went to Iowa City Sunday for a smaller event. Voter turnout in this key political city, which gave a huge assist to then-Senator Barack Obama in the 2008 caucus, will be pivotal for Sanders as well.

239,872 people turned out on the Democratic side for the 2008 caucus, in which Obama beat Clinton, compared to only 124,331 who showed up for the 2004 caucus.

Sanders is not expecting Monday’s caucus to match that 2008 turnout. But he said that he expects voter turnout to be high enough to give him a victory.

Hear the interview: