The former lieutenant governor made the announcement in a video posted on his Facebook page as democratic socialist Francesca Hong has garnered strong support from the liberal wing of the party.
Barnes is the third Democrat to withdraw from the race. Lt. Gov. Sara Rodriguez ended her run amid a campaign finance scandal two weeks ago and former state economic development director Missy Hughes dropped out in July.
After Rodriguez ended her campaign, Milwaukee County Executive David Crowley rejoined the race with the endorsement of Gov. Tony Evers.
The primary is Aug. 11. The winner is expected to face U.S. Rep. Tom Tiffany, who faces only nominal opposition in the Republican primary.
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