MUSCATINE, Iowa — In the days before Christmas, Democratic presidential candidate Bernie Sanders criss-crossed some of the reddest parts of Iowa looking for Donald Trump voters, especially men, to join his progressive political crusade.

Sanders has attacked Trump on the trail in Iowa, on a Sunday TV appearance, and in Twitter back-and-forths. It’s an often-repeated process in the 2016 cycle, often used by flailing Republican candidates trying to get some earned media traction in a cycle dominated by Trump.

But for Sanders, aides say, the fight with Trump is about a central plank of Sanders’ campaign. It’s not often these days that one hears a Democratic politician make a direct play for older white male voters, but for Sanders, it’s becoming an increasing part of his rallying cry.

Last Tuesday, in a crowded auditorium at Buena Vista University in Storm Lake, Iowa, Sanders had a message for working-class men looking to Trump: The Donald is not the answer.

“People are asking why is it that male worker today, with all of the increase in technology and productivity, why is the median male worker, that guy right in the middle of the economy, making $700 less in inflation-adjusted for dollars then he did 40 years ago. And people are anxious, if you're that guy who's working incredibly long hours, you're wondering why is it, what's going on?” Sanders said. “People are anxious and they are angry. Now what you do rationally is you look at these issues, why is the middle class disappearing. why are real wages coming down? Let's talk about that and let's solve those problems. That's what our campaign is about.”

“What Trump is doing is saying, 'OK, those are the issues, but the problem is Muslims coming into this country,’” Sanders continued, mimicking Trump rhetoric. “‘The problem is, you've got a Latino worker in this country making eight bucks and hour. And that's the reason why the middle class in this country is disappearing.'”

Sanders called Trump “a demagogue” in Storm Lake, as he’s done many times before. At another pre-Christmas stop in Council Bluffs, he called out Trump’s attacks on Hillary Clinton for going to the bathroom during the last Democratic debate.

Taking on Trump is good politics for Democrats, and both of Sanders’ rivals for the Democratic nomination have been known to throw an elbow or two at the billionaire businessman.

But for Sanders, pushing on Trump by speaking directly to Trump’s voters has a different purpose. Sanders has long considered himself the candidate of the working class and excoriated Democrats for losing white, working-class voters. His long career as an independent gives him a special credibility to the legion of upset working class voters turning to Trump, aides say.

On the Sunday after Christmas, Sanders made that pitch directly in an appearance on a morning news show.

"For his working class and middle class support, we can make the case that if we really want to address the issues that people are concerned about,” Sanders said. “We need policies that bring us together, that take on the greed of Wall Street the greed of corporate America and create a middle class that works for all of us rather than an economy that works just for a few."

In New Hampshire, where Sanders remains in the lead according to public polling averages, Sanders campaign strategists genuinely believe they can win over some Trump supporters by speaking to their economic angst. A Sanders campaign source said field teams meet undecided voters all the time who are choosing between Sanders and Trump. The campaign in the Granite State has even written a campaign pitch for volunteers to use when they meet voters like that, which the source insisted is a frequent occurrence.

In Iowa, where Hillary Clinton leads, there’s no such formalized effort to win over Trump supporters. A source with the Iowa Sanders campaign said Sanders’s recent talk of Trump was triggered by Trump’s rhetoric, and will return if and when — “when” is more likely, the source said — Trump says something else inflammatory.

But political observers in Iowa say if a Democratic candidate can flip some of Trump’s supporters here, it’s Sanders. Sherry Toelle, chair of the Democratic Party in Iowa’s Republican-leaning Cass County, told BuzzFeed News at a Sanders event in Harlan, Iowa last week that Sanders has the right language for Trump supporters who are worried about the economy.

“His viewpoints and his stressing the importance of the middle class, and stressing all the importance of all the economic reforms, it’s a good starting point,” she said. “Because that’s what a lot of folks are angry about, the economy.”

Sanders supporters from the white working class say Trump helps Sanders even if Sanders doesn’t take him on directly.

Bob Payne, an older Teamster who stood in the back of a Sanders event in Council Bluffs last week with a sign about pensions, said the more Trump keeps talking, the more opportunities Sanders has to show he has the real answers to the economic fears that he said drive many Trump supporters to his camp.

“I hope he keeps having diarrhea of the mouth!” Payne said of Trump. “It just helps Bernie.”

But even as he goes on the attack against Sanders, Trump has expressed skepticism about the idea that his supporters could shift to a candidate who proudly calls himself a democratic socialist.

“I don’t think there’s too many Bernie Sanders people here,” Trump reportedly said a New Hampshire rally Monday night.