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.

But Sanders isn't just staying in his progressive comfort zone. He had thousands of attendees at a rally in Charleston, S.C., and he is planning on campaigning in both Alabama and Louisiana.

"There is no red state, blue state strategy here," Sanders communications manager Michael Briggs told the Washington Examiner. "[Sanders] believes that the issues he's talking about is stuff that makes sense to people no matter what political party."

In the past Sanders has drawn upon a mainly older, white crowd of progressives, but the campaign was pleased with the youth turnout in Wisconsin. "There were more young people than I would have expected," Briggs said.

Sanders talked up about the decline of the middle class, narrowing the income gap, creating jobs, living wages and making higher education more affordable.

"This campaign is not about Bernie Sanders. It is not about Hillary Clinton or anybody else. It is about you," Sanders said as he surveyed the audience. "It is about putting together a grassroots movement of millions of people to make sure the government works for all of us and not a handful of wealthy campaign contributors."