Bernie Sanders won the first election he ever lost.

It was the late 1950s, and Sanders was still a teenager, running to be class president at James Madison High School in Brooklyn, New York. His platform promised to raise scholarship money for kids in Korea orphaned during the recent war. “It was an unusual thing for a person so young to be involved in,” remembers Larry Sanders, Bernie’s older brother. When the votes were tallied, the future Senator from Vermont fell short and lost, but the outcome set a precedent he would love to repeat on the national stage. The winner adopted the Korean scholarship idea and made it happen.

Half a century later, the populist and self-proclaimed socialist is now 73 years old, and he’s running for president of the United States with a solid shot at second place in the Democratic nomination fight. Win or lose, he will force the frontrunner, Hillary Clinton, to take a serious look at his progressive platform, which resonates with a big chunk of the party’s base. “Today, we stand here and say loudly and clearly, enough is enough!” said Sanders on Tuesday evening at his official campaign kickoff in Vermont. “This great nation and its government belong to all of the people, and not to a handful of billionaires.”

“Here is my promise to you for this campaign,” Sanders continued. “Not only will I fight to protect the working families of this country, but we’re going to build a movement of millions of Americans who are prepared to stand up and fight back.”

For Sanders, who maintains he is running to win, pushing Clinton to the left would be fitting capstone to a lifetime spent agitating from the sidelines of powerful American institutions. As a teenager, he read Karl Marx, and as a college student he organized sit-ins against segregation, worked for a union, protested police brutality and attended the 1963 March on Washington. Throughout that time, the central theme of his life has never wavered. “We were concerned obviously about economic injustice,” says Sanders of his college days. “And we were concerned with the question, ‘How do you make change?’”

Sanders’ education in socialism began at home, in a three-and-a-half room apartment in Flatbush, Brooklyn. His father was a paint salesman from Poland and a high school dropout, and the family lived paycheck-to-paycheck. When Sanders’ father went with his wife to see the play The Death of a Salesman, his father so identified with the underemployed Willy Loman that he broke down in tears. “The lack of money caused stress in my family and fights between my mother and father,” Sanders explained to TIME in an interview this month. “That is a reality I have never forgotten: today, there are many millions of families who are living under the circumstances that we lived under.”

Bernie’s older brother, Larry, was a student at Brooklyn College who would come home and discuss Marx and Freud with the high school kid. They talked about democracy in ancient Greece, and Larry took the young Bernie to local Democratic Party meetings. Bernie followed his older brother to Brooklyn College, but when his mother died unexpectedly young, he left Brooklyn and transferred to the University of Chicago.

In Chicago, Sanders threw himself into activism—civil rights, economic justice, volunteering, organizing. “I received more of an education off campus than I did in the classroom,” Sanders says. By his 23rd birthday, Sanders had worked for a meatpackers union, marched for civil rights in Washington D.C., joined the university socialists and been arrested at a civil rights demonstration. He delivered jeremiads to young crowds. The police called him an outside agitator, Sanders said. He was a sloppy student, and the dean asked him to take a year off. He inspired his classmates. “He knows how to talk to people now,” said Robin Kaufman, a student who knew Sanders in 1960s Chicago, “and he knew how to do it then.” He was a radical before it was cool.

He also met regularly with the Young Peoples Socialist League in the student center, where students talked about nuclear disarmament, former Socialist Party Presidential candidate Eugene V. Debs, the lessons of the Russian revolution, and how to implement socialism, though his vision did not match up with the already faltering Soviet experiment. He talks today about expanding government programs like social security and Medicare, and tuition-free college. “Should the government be running the restaurant across the street?” says Sanders today. “Obviously not!”

The civil rights movement also became a home for him. He became leaders of an NAACP ally called the Congress of Racial Equality at a time when most civil rights activists were black. He was arrested while demonstrating for desegregated public schools in Chicago. (No big deal, says Sanders: “You can go outside and get arrested, too!” he jokes. “It’s not that hard if you put your mind to it.”) He once walked around Chicago putting up fliers protesting police brutality. After half an hour, he realized a police car was following him, taking down every paper he’d up, one by one. “Are these yours?” he remembers the officer telling him, holding up the stack of the fliers.

In his second year at college, Sanders made national news. On a frigid Tuesday afternoon in January, 1962 the 20-year-old from Brooklyn stood on the steps of University of Chicago administration building and railed in the wind against the college’s housing segregation policy. “We feel it is an intolerable situation, when Negro and white students of the university cannot live together in university owned apartments,” the young bespectacled student told the few-dozen classmates gathered there. Then he led them into the building in protest, and camped the night outside the president’s office. It was Chicago’s first civil rights sit-in.

Decades later, Sanders rarely raises his past activism in public. In fact, he generally hates talking about his own story. During a recent interview with TIME, the senator from Vermont sunk deep into a sofa in his office and resigned himself to doing just that. “Too much of media looks at politics as a soap opera,” Sanders said in a deep bass. “I have my views, Ted Cruz has his views, that’s fine: let’s lay them out and let the American people decide.”

That aversion to storytelling is part of what makes Sanders a long shot for the Democratic nomination. He polls at around 15% in the early primary states compared with Hillary Clinton’s 60%. And his longtime aversion to the Democratic Party, which he only just formally joined, will be a headwind, as will explaining his identification with “socialism,” a virtual epithet in American politics. “Don’t underestimate me,” Sanders likes to tell reporters.

People who know Bernie best say that beneath the grumpy prognostications about social inequality and climate change is a softy at heart. A few months after he arrived at the University of Chicago, Sanders went to a center in a rough Chicago neighborhood run by a Quaker service group, the American Friends Service Committee. He ventured out to local apartments, painting walls. Back at the house, the 19-year-old was fascinated by the 2-month-old daughter of the home’s caretakers. His friends say he brings that spirit to politics. “His feeling for people is something he had back then, and it’s something he still has,” says Jim Rader, a friend of Sanders’ who ran the Quaker house in Chicago. “He always had a sympathy for the underdog.”

Sanders has lost six major elections since his race for high school class president. But persistence has brought him to his current post, and he’s seeking to be the oldest candidate ever to go to the White House. His goal, at the very least, is to foist his ideas in the Democratic primary. Now, as before, victory can be seen broadly: He can win the nomination himself, or embed his ideas with the person who does.

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