Photos: Former presidential candidate Bernie Sanders US Sen. Bernie Sanders speaks at a campaign rally in Chicago in March 2019. Sanders, an independent from Vermont, is the longest-serving independent in the history of Congress. Hide Caption 1 of 44

Photos: Former presidential candidate Bernie Sanders Sanders, right, leads a sit-in organized by the Congress of Racial Equality in 1962. The demonstration was staged to oppose housing segregation at the University of Chicago. It was Chicago's first civil rights sit-in. Hide Caption 2 of 44

Photos: Former presidential candidate Bernie Sanders Sanders takes the oath of office to become the mayor of Burlington, Vermont, in 1981. He ran as an independent and won the race by 10 votes. Hide Caption 3 of 44

Photos: Former presidential candidate Bernie Sanders Sanders, right, tosses a baseball before a minor-league game in Vermont in 1984. US Sen. Patrick Leahy, center, was also on hand. Hide Caption 4 of 44

Photos: Former presidential candidate Bernie Sanders In 1987, Sanders and a group of Vermont musicians recorded a spoken-word folk album. "We Shall Overcome" was first released as a cassette that sold about 600 copies. When Sanders entered the US presidential race in 2015, the album surged in online sales. But at a CNN town hall, Sanders said, "It's the worst album ever recorded." Hide Caption 5 of 44

Photos: Former presidential candidate Bernie Sanders Sanders reads mail at his campaign office in Burlington in 1990. He was running for the US House of Representatives after an unsuccessful bid in 1988. Hide Caption 6 of 44

Photos: Former presidential candidate Bernie Sanders In 1990, Sanders defeated US Rep. Peter Smith in the race for Vermont's lone House seat. He won by 16 percentage points. Hide Caption 7 of 44

Photos: Former presidential candidate Bernie Sanders Sanders sits next to President Bill Clinton in 1993 before the Congressional Progressive Caucus held a meeting at the White House. Sanders co-founded the caucus in 1991 and served as its first chairman. Hide Caption 8 of 44

Photos: Former presidential candidate Bernie Sanders Barack Obama, then a US senator, endorses Sanders' Senate bid at a rally in Burlington in 2006. Hide Caption 9 of 44

Photos: Former presidential candidate Bernie Sanders Sanders takes part in a swearing-in ceremony at the US Capitol in January 2007. He won his Senate seat with 65% of the vote. Hide Caption 10 of 44

Photos: Former presidential candidate Bernie Sanders Sanders chats with Dr. John Matthew, director of The Health Center in Plainfield, Vermont, in May 2007. Sanders was in Plainfield to celebrate a new source of federal funding for The Health Center. Hide Caption 11 of 44

Photos: Former presidential candidate Bernie Sanders Sanders speaks to reporters in 2010 about the Obama administration's push to extend Bush-era tax cuts. Three days later, Sanders held a filibuster against the reinstatement of the tax cuts. His speech, which lasted more than eight hours, was published in book form in 2011. It is called "The Speech: A Historic Filibuster on Corporate Greed and the Decline of Our Middle Class." Hide Caption 12 of 44

Photos: Former presidential candidate Bernie Sanders Sanders and US Rep. Jeff Miller, chairman of the House Committee on Veterans' Affairs, walk to a news conference on Capitol Hill in 2014. Sanders was chairman of the Senate Committee on Veterans' Affairs. Hide Caption 13 of 44

Photos: Former presidential candidate Bernie Sanders In March 2015, Sanders speaks in front of letters and petitions asking Congress to reject proposed cuts to Social Security and Medicare. Hide Caption 14 of 44

Photos: Former presidential candidate Bernie Sanders In July 2015, two months after announcing he would be seeking the Democratic Party's nomination for President, Sanders spoke to nearly 10,000 supporters in Madison, Wisconsin. "Tonight we have made a little bit of history," he said. "You may know that some 25 candidates are running for President of the United States, but tonight we have more people at a meeting for a candidate for President of the United States than any other candidate has." Hide Caption 15 of 44

Photos: Former presidential candidate Bernie Sanders Seconds after Sanders took the stage for a campaign rally in August 2015, a dozen protesters from Seattle's Black Lives Matter chapter jumped barricades and grabbed the microphone from the senator. Holding a banner that said "Smash Racism," two of the protesters -- Marissa Johnson, left, and Mara Jacqueline Willaford -- began to address the crowd. Hide Caption 16 of 44

Photos: Former presidential candidate Bernie Sanders Sanders shakes hands with Hillary Clinton at a Democratic debate in Las Vegas in October 2015. The hand shake came after Sanders' take on the Clinton email scandal. "Let me say something that may not be great politics, but the secretary is right -- and that is that the American people are sick and tired of hearing about the damn emails," Sanders said. "Enough of the emails, let's talk about the real issues facing the United States of America." Hide Caption 17 of 44

Photos: Former presidential candidate Bernie Sanders Sanders embraces Remaz Abdelgader, a Muslim student, during an October 2015 event at George Mason University in Fairfax, Virginia. Asked what he would do about Islamophobia in the United States, Sanders said he was determined to fight racism and "build a nation in which we all stand together as one people." Hide Caption 18 of 44

Photos: Former presidential candidate Bernie Sanders Sanders waves while walking in a Veterans Day parade in Lebanon, New Hampshire, in November 2015. Hide Caption 19 of 44

Photos: Former presidential candidate Bernie Sanders Sanders sits with rapper and activist Killer Mike at the Busy Bee Cafe in Atlanta in November 2015. That evening, Killer Mike introduced Sanders at a campaign event in the city. "I'm talking about a revolutionary," the rapper told supporters. "In my heart of hearts, I truly believe that Sen. Bernie Sanders is the right man to lead this country." Hide Caption 20 of 44

Photos: Former presidential candidate Bernie Sanders Comedian Larry David and Sanders appear together on "Saturday Night Live" in February 2016. David had played Sanders in a series of sketches throughout the campaign season. Hide Caption 21 of 44

Photos: Former presidential candidate Bernie Sanders Sanders and his wife, Jane, wave to the crowd during a primary night rally in Concord, New Hampshire, in February 2016. Sanders defeated Clinton in the New Hampshire primary with 60% of the vote, becoming the first Jewish candidate to win a presidential primary. Hide Caption 22 of 44

Photos: Former presidential candidate Bernie Sanders Sanders speaks at a campaign rally in Ann Arbor, Michigan, in March 2016. He won the state's primary the next day, an upset that delivered a sharp blow to Clinton's hopes of quickly securing the nomination. Hide Caption 23 of 44

Photos: Former presidential candidate Bernie Sanders Sanders speaks at a campaign event in New York's Washington Square Park in April 2016. Hide Caption 24 of 44

Photos: Former presidential candidate Bernie Sanders Sanders speaks at a rally in Santa Monica, California, in June 2016. He pledged to stay in the Democratic race even though Clinton secured the delegates she needed to become the presumptive nominee. Hide Caption 25 of 44

Photos: Former presidential candidate Bernie Sanders Sanders endorses Clinton at a rally in Portsmouth, New Hampshire, in July 2016. Hide Caption 26 of 44

Photos: Former presidential candidate Bernie Sanders Sanders addresses delegates on the first day of the Democratic National Convention in July 2016. Hide Caption 27 of 44

Photos: Former presidential candidate Bernie Sanders Sanders brings a giant printout of one of Donald Trump's tweets to a Senate debate in January 2017. In the tweet, Trump had promised not to cut Social Security, Medicare and Medicaid. Hide Caption 28 of 44

Photos: Former presidential candidate Bernie Sanders Sanders thanks supporters after winning re-election to the Senate in November 2018. Hide Caption 29 of 44

Photos: Former presidential candidate Bernie Sanders Sanders looks at his notes as he watches President Trump deliver the State of the Union address in February 2019. That month, Sanders announced that he would be running for president again. Hide Caption 30 of 44

Photos: Former presidential candidate Bernie Sanders Sanders hugs a young supporter during a campaign rally in Los Angeles in March 2019. Hide Caption 31 of 44

Photos: Former presidential candidate Bernie Sanders Sanders addresses the audience at a CNN town hall in Washington in April 2019. Hide Caption 32 of 44

Photos: Former presidential candidate Bernie Sanders Sanders speaks next to former Vice President Joe Biden at the first Democratic debates in June 2019. Hide Caption 33 of 44

Photos: Former presidential candidate Bernie Sanders Sanders raises his fist as he holds a rally in Santa Monica, California, in July 2019. Hide Caption 34 of 44

Photos: Former presidential candidate Bernie Sanders Sanders grabs the hand of US Sen. Elizabeth Warren during the Democratic debates in Detroit in July 2019. Hide Caption 35 of 44

Photos: Former presidential candidate Bernie Sanders Sanders campaigns at the University of New Hampshire in September 2019. A few days later, he took himself off the campaign trail after doctors treated a blockage in one of his arteries. Sanders suffered a heart attack, his campaign confirmed. Hide Caption 36 of 44

Photos: Former presidential candidate Bernie Sanders US Rep. Alexandria Ocasio-Cortez introduces Sanders at a New York rally after endorsing him for president in October 2019. Hide Caption 37 of 44

Photos: Former presidential candidate Bernie Sanders In a tense and dramatic exchange moments after a Democratic debate, Warren accused Sanders of calling her a liar on national television. Sanders responded that it was Warren who called him a liar. Earlier in the debate, the two disagreed on whether Sanders told Warren, during a private dinner in 2018, that he didn't believe a woman could win the presidency. Hide Caption 38 of 44

Photos: Former presidential candidate Bernie Sanders Sanders laughs during a primary-night rally in Manchester, New Hampshire, in February 2020. Sanders won the primary, just as he did in 2016. Hide Caption 39 of 44

Photos: Former presidential candidate Bernie Sanders A triumphant Sanders raises his fist in San Antonio after he was projected to win the Nevada caucuses. Hide Caption 40 of 44

Photos: Former presidential candidate Bernie Sanders Sanders and former Vice President Joe Biden talk before a Democratic debate in Charleston, South Carolina, in February 2020. Hide Caption 41 of 44

Photos: Former presidential candidate Bernie Sanders Sanders addresses supporters during a campaign rally in Grand Rapids, Michigan, in March 2020. Hide Caption 42 of 44

Photos: Former presidential candidate Bernie Sanders Sanders speaks to reporters in Burlington, Vermont, a day after Super Tuesday II. Sanders said it "was not a good night for our campaign from a delegate point of view" but that he looked forward to staying in the race and taking on Joe Biden in an upcoming debate. Hide Caption 43 of 44