Wednesday's rally was Sanders' largest yet, and may be the biggest of the 2016 cycle overall. Clinton's campaign launch drew approximately 5,500 people to New York City's Roosevelt Island, while about 3,000 supporters attended Jeb Bush's kickoff in Miami.

Sanders' progressive messages resonated strongly in Wisconsin's liberal, capital city. "The big money interests — Wall Street, corporate America, all of these guys — have so much power that no president can defeat them unless there is an organized grassroots movement making them an offer they can't refuse," he said as the crowd erupted in cheers, the AP reported.

"When you deny the right of workers to come together in collective bargaining, that's extremism," Sanders said, going after Wisconsin Gov. and White House hopeful Scott Walker. "When you tell a woman that she cannot control her own body, that's extremism.