Sen. Bernie Sanders, 78, has surged ahead in the 2020 Democratic primary election, running toe-to-toe with former Vice President Joe Biden.

Sanders took the lead in early voting after winning the first three states: Iowa, New Hampshire, and Nevada. He has since fallen slightly behind Biden, who dominated the vote on Super Tuesday.

For nearly four decades, Sanders has been fighting against the people and institutions he views as responsible for rigging the system against the middle class.

Once viewed on the fringe of politics, Sanders' democratic socialist platform has taken a stronghold since he first ran for president in 2016.

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Sen. Bernie Sanders announced his candidacy for president in February 2019, following a 2016 campaign run that brought him to the forefront of the political conversation in the US.

Since entering the race, Sanders has quickly gained traction. In February, he became the first presidential candidate in history to win all three early voting states, and despite swearing off of private fundraising, he has raised more money than any other candidate.

Claiming the mantle of democratic socialism, Sanders has railed against a consistent set of targets: Wall Street, multinational corporations, the billionaire class, and the political elite. Throughout his career, he's fought them for ganging up to "rig the system" against working class Americans.

As mayor of Burlington, Vermont, then a House Representative, and later a US senator from the state, he's repeatedly assailed the "establishment", calling for sweeping reforms to remake the nation's economy in a more Scandinavian image.

Sanders has popularized progressive reform such as universal healthcare, increased minimum wage, and debt-forgiveness for college students. Today, many Democratic candidates are running on ideas Sanders championed.

The Vermont senator has long been guarded about his personal life, instead opting to sell his vision of wholesale revolution on the campaign trail. But he's shared more details about his early life in Brooklyn in an effort to connect with voters amid a crowded primary where he is not the only progressive firebrand running for president.

Here's how Sanders went from being a working-class kid in Brooklyn to a top 2020 Democratic presidential candidate.