Democratic presidential candidates accused each other of support for authoritarianism during their debate in South Carolina, with Michael Bloomberg taking heat for his comments on China’s Xi Jinping, and Bernie Sanders coming under fire for recent comments praising Cuba’s education system.

Former New York Mayor Mike Bloomberg defended his stance on China, saying that President Xi has to answer to China’s Politburo, and arguing that the U.S. must engage with China.

This drew a sharp response from former vice president Joe Biden, who said that Xi is an un-democratic ruler, but one who must be negotiated with.

“This is a guy who doesn’t have a democratic with a small d, bone in his body. This is a guy who is a thug who in fact, has a million Uyghurs in reconstruction camps, meaning concentration camps,” Biden said.

Vermont Senator Bernie Sanders was attacked by several candidates over recent comments praising the literacy program in Cuba begun under former Cuban leader Fidel Castro. Sanders defended himself by pointing to similar comments by former President Barack Obama.