"Given your large readership (the Los Angeles Times, fourth-largest circulation in the country; the Chicago Tribune, ninth-largest circulation; Hoy, second-largest Spanish-language daily newspaper in the country), it concerns us that one of the potential buyers, Koch Industries, is interested in the purchase of your print media outlets," the letter, sent to Peter Liguori, the CEO of Tribune Company, reads.

Democracy for America sent out a copy of the email to supporters on Friday. Koch Industries is owned by Charles and David Koch, billionaires that fund a number of conservative and libertarian causes.

In late April, a number of news outlets reported that Koch Industries had begun considering a bid to buy Tribune Company's regional newspapers.

Sanders and Dean describe the Kochs as "right-wing extremists" with "enormous economic and political power."

"They have also established and funded dozens of organizations that actively support the privatization of Social Security, massive cuts in programs for working families, attacks on workers' rights, more tax breaks for billionaires and large corporations, and other causes benefitting the rich and powerful," Dean and Sanders write.

As the company picks a buyer, Dean and Sanders conclude in their letter, it should quickly reject any bid from Koch Industries.

"We are writing, therefore, to voice our opposition to the Tribune Company selling any of its media outlets to Koch Industries," Dean and Sanders conclude in the letter. "As you begin to narrow down potential buyers, we urge you to reject any offer from Koch Industries."