U.S. Democratic presidential candidate Bernie Sanders attends a campaign rally at the Petrillo Music Shell in Grant Park of Chicago, the United States, on March 7, 2020. (Xinhua/Wang Ping)

U.S. Democratic presidential candidate Bernie Sanders speaks during a campaign rally at the Petrillo Music Shell in Grant Park of Chicago, the United States, on March 7, 2020. (Xinhua/Wang Ping)

U.S. Democratic presidential candidate Bernie Sanders speaks during a campaign rally at the Petrillo Music Shell in Grant Park of Chicago, the United States, on March 7, 2020. (Xinhua/Wang Ping)

U.S. Democratic presidential candidate Bernie Sanders attends a campaign rally at the Petrillo Music Shell in Grant Park of Chicago, the United States, on March 7, 2020. (Xinhua/Wang Ping)

U.S. Democratic presidential candidate Bernie Sanders attends a campaign rally at the Petrillo Music Shell in Grant Park of Chicago, the United States, on March 7, 2020. (Xinhua/Wang Ping)

U.S. Democratic presidential candidate Bernie Sanders attends a campaign rally at the Petrillo Music Shell in Grant Park of Chicago, the United States, on March 7, 2020. (Xinhua/Wang Ping)