Democrat presidential candidate Bernie Sanders wrote a fiery letter to the liberal Center for American Progress (CAP) this week, charging that the activist group is effectively instigating a civil war among prominent Democrats that could end up ensuring President Trump's re-election.

Sanders also issued a thinly veiled threat against CAP, warning that he "will be informing my grassroots supporters of the foregoing concerns that I have about the role CAP is playing" and that "should your actions evolve in the coming months, I am happy to reconsider what kind of partnership we can have."

The missive comes days after CAP's ThinkProgress branch published a YouTube video mocking Sanders for spending years attacking "millionaires and billonaires" -- only to change his tune and only criticize "billonaires" shortly after he wrote a bestselling book that made him a millionare. The video prompted a frenzy of social media among progressives as to why a progressive group would try to take down Sanders.

In the letter, first obtained by The New York Times, Sanders said ThinkProgress' in its video "honestly attacked me for hypocrisy in my effort to address income inequality in America." Sanders also said the organization wrote an article mocking him for his "appearance and for the income I earned from writing a book."

Sanders also accused CAP of going after Democrat candidates Cory Booker and Elizabeth Warren.

"I and other Democratic candidates are running campaigns based on principles and ideas and not engaging in mudslinging or personal attacks on each other," Sanders wrote.

"Meanwhile, the Center for American Progress is using its resources to smear Senator Booker, Senator Warren, and myself, among others," he added. "This is hardly the way to build unity, or to win the general election."

Sanders has attracted widespread criticism from progressives and conservatives alike for telling the Times on Tuesday: "If you write a best-selling book, you can be a millionaire, too."

At a rally later in Indiana, Sanders doubled down, telling attendees Saturday, "I didn't know that it was a crime to write a good book, which turned out to be a best-seller."

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The progressive infighting comes after public squabbling from former members of Hillary Clinton and Sanders' teams, over everything from Sanders' penchant for expensive private air travel to Clinton's purported lack of gratitude.

Sanders was on the friendly terrain of Wisconsin on Friday, promising to build a coalition that will defeat Trump as he kicked off a swing through pivotal states that are part of the Democratic "blue wall" strategy for 2020.

Sanders, speaking to a crowd of about 2,400 who braved 40-degree temperatures with a stiff 20 mph wind, pledged to flip Midwestern states such as Wisconsin that Trump narrowly won in 2016.

"Together, we are going to make sure that does not happen again," Sanders said to cheers. "We're going to win here in Wisconsin. We're going to win in Indiana. We're going to win in Ohio. We're going to win in Michigan. We're going to win in Pennsylvania and together we're going to win this election."

Sanders spoke in a city park along the shores of a lake, just a few blocks away from the state Capitol. Those in the crowd wore ski masks, winter coats, gloves and scarves as they cheered Sanders.

The Independent senator from Vermont carried Wisconsin by 13 points in the Democratic primary three years ago and has been a frequent visitor since losing the nomination to Hillary Clinton. He touted his appeal to working-class and college-age voters, while fostering his network of supporters before this second run for president.

Democrats have made clear that their best chance at defeating Trump in 2020 is by winning back three states Trump narrowly captured: Wisconsin, Michigan and Pennsylvania. Dubbed the "blue wall" before they unexpectedly tipped to Trump, they may have supplanted Florida and Ohio as the nation's premier presidential battlegrounds.

Following the Wisconsin rally, Sanders went to Gary, Indiana, on Saturday. He held a rally at a community college in Warren, Michigan, and is heading to Pennsylvania for an event Sunday near the University of Pittsburgh and Carnegie Mellon campuses. Then it's off to Ohio on Monday.

Trump also knows the Midwest is vital to his re-election bid. He's looking to repeat in states he won in 2016 and expand his territory. Trump was due to campaign Monday in Minnesota, a state that almost went his way in 2016 after not voting for a Republican presidential candidate since Richard Nixon in 1972.

However, Democrats feel like the momentum is on their side in the Midwest. They captured governorships in Pennsylvania, Michigan, Wisconsin, Illinois and Minnesota last year.

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Pollster Charles Franklin cautioned about reading too much into the numbers a full year before Wisconsin's April 2020 primary. Candidates with the best name recognition, like Sanders, typically fare better this far out compared with others mounting their first national runs for office, Franklin said.

The Sanders campaign said his message on trade, unions, working families and health care resonates in Wisconsin and throughout the Midwest. Earlier this week, Sanders unveiled his latest "Medicare-for-All" proposal, an idea that has influenced Democratic state lawmakers in Wisconsin who are advocating for similar statewide health insurance coverage.

Fox News' Brooke Singman and The Associated Press contributed to this report.