The Associated Press called the open primary just after 8pm on Tuesday. Feingold defeated businessman Scott Harbach of Kenosha to make it onto the ticket.
"I'm incredibly grateful for the thousands of Wisconsinites who voted in today's primary, and I'm proud to accept the Democratic nomination to serve the people of this state in the U.S. Senate," Feingold said in a statement following the vote.
Feingold represented Wisconsin in the Senate for 18 years before being ousted during a 2010 Tea Party wave that elected Johnson to his seat. In 2015, however, Politico described Johnson as "one of the most vulnerable incumbents on the 2016 Senate map."
On Tuesday, Feingold told volunteers in Racine that his campaign wants to address economic inequality in Wisconsin, where the rightwing billionaire Koch brothers have long had undue political influence.
"The fundamental issue is that the very wealthy interests...the corporations, the billionaires, the multimillionaires have taken over this country and our democracy," Feingold said, adding that the Koch brothers have staged "an unfriendly takeover of the state of Wisconsin."
"We're going to turn that around," he said.
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The Milwaukee Journal Sentinel reports:
For months, Feingold has held a consistent lead in the Marquette University Law School Poll. In June, the poll showed Feingold leading Johnson by 7 percentage points among registered voters and 5 points among likely voters. The poll releases its latest findings Wednesday.
"Wisconsinites now have a clear choice in this election. Sen. Johnson has spent years in Washington ignoring the people of this state. Instead he's fought to protect a system that benefits multi-millionaires like himself, oil billionaires, powerful corporations and the special interests funding the super PACs dedicated to his reelection bid," Feingold said.
"Wisconsinites deserve an economy that works for everyone—not just corporate CEOs and the special interests. Wisconsinites deserve leaders who will actually listen to and fight for them."
Feingold announced his re-election campaign in 2015. At the time, National Journal political editor Josh Kraushaar noted that he was the Democrats' "leading campaign finance scold" while in the Senate—and he "lived up to his principles in practice."
"Feingold is a favorite of progressives, and his candidacy would be a reliable way to energize the grassroots base," Kraushaar wrote.