No American politician had a more polarizing effect on voters last year than Scott Walker. This time last year, thousands of irate protesters were occupying Wisconsin’s state Capitol, comparing Mr. Walker to Hitler for trying to reform the pension and collective-bargaining systems of public-employee unions. He needed an entourage of 25 security officers to escort him through the building at the height of the pandemonium. …

So now Mr. Walker, his lieutenant governor and four state senators will face the voters as early as this spring or summer. Last year a similar effort targeted four other GOP state senators and a Republican on the state supreme court. After unions spent millions on the campaign, two of the state senators were recalled but that failed to flip control of the legislature, and the supreme court justice kept his seat.

A problem for Democrats this year is finding an A-list candidate willing to run against Mr. Walker if he is recalled. A strong opponent could be Milwaukee Mayor Tom Barrett, but the unions are furious with him too for implementing many of the Walker union reforms—saving his city $25 million and balancing its budget in the process. The likeliest candidates to challenge Mr. Walker are two liberals, former Congressman David Obey and Madison County Executive Kathleen Falk.