On May 1, left-wing vigilantes will target companies across the country that have committed a mortal sin: sending donations to GOP Gov. Scott Walker of Wisconsin. Rest assured, such intolerable acts of political free speech will not go unpunished by tolerant Big Labor activists. They’re calling for both a national boycott of Walker’s corporate donors and a coordinated sticker vandalism campaign on GOP-tainted products.

The Wisconsin Grocers Association is bracing for the anti-Walker witch hunt. Anonymous operatives have circulated sabotage stickers on the Internet and around Wisconsin that single out Angel Soft tissue paper (“Wiping your (expletive) on Wisconsin workers”), Johnsonville Sausage (“These Brats Bust Unions”) and Coors (“Labor Rights Flow Away Like A Mountain Stream”). Earlier this week, a “Stick It To Walker” website boasted photos of vandalized Angel Soft tissue packages at a Super Foodtown grocery store in Brooklyn, N.Y.

This destruction of private property is illegal. Not that it matters to anti-Walker protest mobsters, who trampled Wisconsin’s Capitol at an estimated $5 million in security, repair and cleaning costs to taxpayers. According to the Milwaukee Journal Sentinel, “The identity of the backers of the sticker effort is unknown, although many assume it is being orchestrated by public employee unions. This latest effort follows boycotts organized by members of the Wisconsin State Employees Union AFSCME 24.”

AFSCME 24 is the same union affiliate that recently disseminated intimidation letters throughout southeast Wisconsin, demanding that local businesses support unions by putting up signs in their windows. The letter threatened not just Walker supporters, but any and all businesses that have chosen to sit on the sidelines and stay out of politics altogether: “Failure to do so will leave us no choice but (to) do a public boycott of your business. And sorry, neutral means ‘no’ to those who work for the largest employer in the area and are union members.”

Others on Big Labor’s hit list: Kwik Trip, Sargento Foods Inc. and M&I Bank.

Walker, of course, has been at the forefront of government pension and budget reforms. Similar measures are being advanced by Democratic governors and Democrat-run legislatures from Massachusetts to New York to California. But union bosses have yet to sic their goons on individual and corporate donors to Democratic politicians imposing long-overdue benefit and collective bargaining limits for public employee unions.

How convenient, yes? Just as they secured a big fat waiver from the federal health care mandate and tax scheme they lobbied to impose on the rest of America, Big Labor is giving Democratic legislative water-carriers who have been forced to adopt cuts and cost controls a big fat waiver from their organized wrath and vandalism.

Now, a few hundred or thousand ruined grocery store items may not seem to matter much to the average reader, but this little property destruction campaign spotlights a nasty tactic increasingly employed by the left: campaign finance disclosure as a speech-squelching weapon.

We saw it last fall when Democratic operatives targeted the U.S. Chamber of Commerce for donating to Obamacare opposition ads.

We saw it in 2008 when a top MoveOn.org alumnus launched attacks on Republican donors with the express purpose of “hoping to create a chilling effect that will dry up contributions.”

We saw it when Obama campaign committee lawyers lobbied the Justice Department to investigate and prosecute a GOP donor for funding campaign ads exposing Obama’s ties to Weather Underground terrorist Bill Ayers.

We saw it during the Proposition 8 traditional marriage battle in California, where gay rights avengers compiled black lists, harassment lists and Google target maps of citizens who contributed to the ballot measure.

We saw it when “progressive” zealots smeared Target Corporation and Chick-fil-A for daring to associate with social conservatives.

And we’re seeing it again this month as the Obama White House readies an executive order that would force federal contractors to disclose all political donations to candidates and independent groups in excess of $5,000 made not just by a corporate entity, but by all of its individual executives, directors and officers.

Former Federal Election Commission official Hans von Spakovsky obtained the sweeping draft executive order, which — surprise, surprise — exempts unions and predominantly left-wing federal grant recipients from the mandate. On Wednesday, GOP senators spelled out the bullying agenda in an open letter objecting to the Obama order: “Political activity would obviously be chilled if prospective contractors have to fear that their livelihood could be threatened if the causes they support are disfavored by the administration.” Join the club.

When disclosure’s a bludgeon, all but Obama’s cronies are nails.

Michelle Malkin is the author of “Culture of Corruption: Obama and his Team of Tax Cheats, Crooks & Cronies” (Regnery 2010).