Former Sen. Russ Feingold (D-Wis.) is already raising money for Milwaukee Mayor Tom Barrett, who won a Democratic gubernatorial primary election on Tuesday and will face off against Gov. Scott Walker (R) in the June 5 general election.

From the email he sent to supporters of his group, Progressives United, on Wednesday morning, asking for $25 donations:

Mayor Barrett is a strong standard-bearer for all Wisconsinites who have seen their rights trampled and their livelihoods threatened by Walker and the Koch brothers' corporate schemes. And while Mayor Barrett's strength lies in the grassroots, Scott Walker still relies on the corporate bosses from coast to coast who armed him with tens of millions of dollars from the get-go.

But if people like you unite with fellow progressives around Tom Barrett, we can put an end to Walker's harmful policies -- and slam the brakes on the national corporate agenda. [...]

Moving forward, Mayor Barrett has the ability to help heal this divided state and bring true leadership to Madison. His strong executive experience makes him qualified to begin rolling back Walker's devastating program on day one.

Tom is a lifelong progressive whose record speaks for itself. While representing Wisconsin in Congress, Mayor Barrett stood with me in voting against the deregulation that led to the Wall Street crash, opposing the PATRIOT Act, and reforming our system of campaign finance.

Feingold remains a popular progressive figure since losing his Senate reelection campaign in 2010, and he has been active in protesting and speaking out against Walker and his Republican allies in the state legislature. Indeed, polls last year showed he would have perhaps been the strongest Democrat in the state to go up against Walker.

Feingold sending out an email to his list will not only help raise money to defeat Walker, but it's an attempt to unify progressives -- some of whom may have supported one of Barrett's three Democratic challengers -- a day after the primary election. Former Dane County executive Kathleen Falk, in particular, had the strong backing of many labor unions and progressive groups.

Democrats are also holding "unity" events in Wisconsin on Wednesday, hoping to bring the party together.

Also on HuffPost: