Sen. Bernie Sanders (I-Vt.) continues to endure a series of unflattering reports about past words and beliefs, with the latest coming from the Daily Beast, which reported that in the 1970s, Sanders repeatedly compared Vermont workers to African slaves.

In 1976, when Sanders was the gubernatorial nominee of the Liberty Union Party in Vermont, he railed against a decision by private owners of Vermont Marble, a mining company, to sell to a Swiss conglomerate without the consent of employees.

"We believe ultimately that companies like Vermont Marble should be owned by the workers themselves and that workers — not a handful of owners — should be determining policy," Sanders said, according to media reports obtained by the Daily Beast. "If a worker at Vermont Marble has no say about who owns the company he works for and that major changes can take place without his knowledge and consent, how far have we really advanced from the days of slavery, when black people were sold to different owners without their consent?"

The next year, Sanders was the chairman of the Liberty Union Party after losing the governor's race and earning only 6 percent of the vote. He was critical of Vermont's growing service industry, and again used a slavery comparison to describe service industry workers.

"How can a worker be happy with his or her job when he or she has no control over that job?" Sanders asked in a 1977 interview. "The problem comes when we end up with an entire state of people trained to wait on other people. ... basically, today, Vermont workers remain slaves in many, many ways."

One of the primary weaknesses of Sanders' campaign in 2016 was his inability to attract support from black voters. He is regularly criticized for viewing racial inequality through a purely economic lens, rather than acknowledging the historical and cultural factors that contribute to the problem.

Sanders isn't the only Democratic front-runner with problematic racial comments from decades past. In the months before he launched his presidential campaign, Joe Biden was criticized for speaking favorably of segregationist politicians and for opposing federally-mandated busing as a way to desegregate schools.