The Vermont senator recalled the incident that occurred this past Thursday evening at Veterans Memorial Coliseum in Phoenix where a man unfurled a flag containing the Nazi symbol. Sanders’ supporters took control of the situation, taking the flag away from the the man. He was shortly removed by security afterwards.

“He was behind me,” Sanders told Jake Tapper on the program State of the Union. And I was speaking to the crowd and I saw crowds booing and I turned around, I didn’t quite see what it was. I learned about it right after I left the stage. The idea that there was a swastika, a symbol of everything that this country stands against – we lost 400,000 people fighting that symbol, fighting Nazism."

Sanders, who is Jewish, was quick to remind of the horror that transpired under that symbol.

“Six million Jews were killed, other people were killed. The most devastating war in the history of humanity. Obviously, it is unspeakable. It is disgusting. It is something – I got to tell you, I never expected in my life, as an American, to see a swastika at a major political rally. It’s horrible,” Sanders continued.

Support from Sanders’ colleagues, staff and supporters quickly flooded social media following the incident. His primary opponent in the Democratic race, Joe Biden, quickly denounced the act.

“I don’t care who you’re supporting, attacks like this against a man who could be the first Jewish President are disgusting and beyond the pale. Hatred and bigotry have no place in America — and it’s up to all of us to root out these evils wherever they’re found,” Biden said on Twitter.

Recently, then-MSNBC host Chris Matthews made a remark in February that compared Sanders’ Nevada caucuses’ victory to the 1940 Nazi invasion of France: “I was reading last night about the fall of France in the summer of 1940 and the general, Reynaud, calls up Churchill and says, ‘It’s over.’ And Churchill says, ‘How can that be? You’ve got the greatest army in Europe. How can it be over?’ He said, ‘It’s over.’”

That comment sparked outcry, especially from the American Jewish progressive activist group IfNotNow. “Comparing Bernie’s progressive vision, where everyone has the right to healthcare, education, and a good paying job to the rise of fascism and impending Holocaust in Europe is incredibly offensive,” said the group’s communications director Yonah Lieberman. “It’s even worse when you factor in the fact that Bernie has family members that were killed in the Holocaust. And it’s even worse than that when you know that there is a growing white nationalist movement in America right now.”

Matthews apologized for the comment prior to stepping down from MSNBC last week. “Senator Sanders,” he said at the time, “I’m sorry for comparing anything from that tragic era in which so many suffered, especially the Jewish people to an electoral result in which you were the well-deserved winner.”