Democratic underdog Bernie Sanders has now held the biggest rally of the 2016 presidential campaign.

Nearly 10,000 people attended the Vermont senator's event in Madison, Wis., Wednesday night, the highest turnout for the campaign so far. The self-proclaimed socialist has surged in the polls over the past few weeks, posing himself as the only credible threat to Democratic frontrunner Hillary Clinton.

"Tonight we have made a little bit of history," Sanders told the crowd before his hour-long speech. "Tonight, we have more people at any meeting for a candidate of president of the United States than any other candidate."

Rather than hold small-scale meet-and-greets in local venues in early primary states, Sanders has focused on creating grassroots momentum in classically liberal cities. A rally in Burlington, Vt., attracted 5,000 people, 5,000 showed up in Denver and 3,000 came to one in Minneapolis.

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