IOWA CITY, Iowa — A leader of Bernie Sanders's campaign in Iowa told staff members and volunteers to be aware of leaders within the Democratic Party who want to stop the momentum of the Vermont senator's White House run.

"As you can see, when power is threatened, power is resilient. When power is threatened, it responds," said Stacey Walker, the Sanders campaign Iowa co-chairman at a staff meeting Friday afternoon. "We're seeing that right now on the national level with very important establishment voices trying to warn the rest of their people about this growing movement. But this movement is also strong."

Walker, who was the first black supervisor elected to the board of Linn County, Iowa, also worked for President Barack Obama's 2012 reelection and for the Obama-affiliated political action committee Organizing for Action.

"We can look at the Republican Party and the Democratic Party and call out the bullshit. That's what it's gonna take. In order for big change to happen, you have to have an honest conversation about what's actually wrong," Walker said. "You can't have an honest conversation if we're doing what the ruling class wants," and he later said he believed Sanders would win the Iowa caucuses on Feb. 3.

Walker's comments come as members of the party's establishment have grown increasingly concerned that Sanders may end up their presidential nominee, particularly as national polls show Sanders overtaking former Vice President Joe Biden for first place. A win in Iowa, many moderate members of the party worry, could create a domino effect that gives Sanders the momentum to lock the primary up early on.

Those who are openly critical of Sanders include former Secretary of State Hillary Clinton, who declined to say in an interview earlier this week that she would back Sanders in a general election.

"He was in Congress for years. He had one senator support him. Nobody likes him. Nobody wants to work with him. He got nothing done. He was a career politician. It's all just baloney, and I feel so bad that people got sucked into it,” she said.

Clinton later walked back those comments and said she's looking forward to helping any Democrat unseat Trump in November.

Last year, reports surfaced that Obama considered publicly stepping into the primary in order to thwart Sanders. Obama reportedly told confidantes that he would feel obligated "to speak up and stop him."

Obama's former campaign manager, Jim Messina, has repeatedly criticized Sanders's campaign as doomed to fail in a general election. In an MSNBC interview Thursday, Messina called Sanders "the worst candidate."

"I think it's very clear to me that these swing voters, that I care about, the Trump-Obama voters in midwestern states, Bernie Sanders is not the candidate we need to beat Donald Trump in November," he said.