Presidential candidate Sen. Bernie Sanders (I) of Vermont joined the Democratic race for the White House as a long shot, but he continues gaining momentum since he first emerged as Hillary Clinton’s biggest primary challenger.

The self-described "democratic socialist" has been gaining ground on the front-runner in Iowa, an important early marker of primary success. His support has more than doubled since May, with 33 percent of Democratic caucus-goers in the state favoring the Vermont senator, compared with 52 percent for Clinton, according to a new Quinnipiac poll.

A stop in Wisconsin on Wednesday garnered his biggest crowd to date, with 10,000 people packing the Veterans Memorial Coliseum in Madison.

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

His populist campaign messaging has remained the same, hammering home points about income inequality, raising the minimum wage, and criminal justice reform. In Madison, Sanders went after Wisconsin's Republican Gov. Scott Walker in his speech, attacking him for restricting labor rights and his conservative agenda.

"When you deny the right of workers to come together in collective bargaining, that's extremism," Sanders said to enthusiastic crowd. "When you tell a woman that she cannot control her own body, that's extremism."

Sanders has built his supporter base by tapping into an undercurrent of anger and discontent about economic inequality and the influence of big business in politics.

"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," Sanders said.

Governor Walker is expected to make his own announcement for a White House bid later this month and released a statement bashing Sanders and his policies ahead of his visit to the Badger State.

"Bernie Sanders is right about one thing: We don't need another Clinton in the White House," Walker said. "On virtually every other issue, however, he stands in stark opposition to most Americans. Wisconsinites have rejected his top-down, government-knows-best approach three times in the last four years."

Sanders has been criss-crossing the country raising support and funding from liberal voters in Democratic stronghold states. A Minnesota campaign event scheduled for Thursday is his second foray in the state, which is not an early primary battleground and indicates he’s in the race for the long haul.

At his last stopover in the Land of 10,000 Lakes, Sanders was able to attract an audience of 3,000 by touting his progressive credentials and left-wing ideas.

Sanders has been drawing some of the largest crowds of supporters early this campaign season, but his challenge during the upcoming election is turning this vocal support into actual voters at the booth.

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“I do not believe that any president who’s standing up for the working class of this country can be successful without a mobilized, activist, grass-roots movement behind him or her,” Sanders said at a Monitor breakfast last month. “So I will be working hard to make sure that that mobilization exists.”

[Editor's note: The original version misidentified Sanders's political affiliation. He is an independent running for the Democratic presidential nomination.]