Holman W. Jenkins Jr.:

In Thursday’s Republican debate, Wisconsin Gov. Scott Walker will be the antiunion candidate. That will be the media snark. He signed (unenthusiastically) a right-to-work bill that applies to his state’s private-sector workers. He promoted (very enthusiastically) a law that all but ended collective bargaining for its public-sector workers.

Critics will ask: What does this have to do with being president? Unfortunately, everything.

Unions may not matter much in American workplaces anymore but unions represent the main political obstacle to just about every kind of reform: School choice. Entitlements. Pensions. Health care.

Even causes that wouldn’t seem union business prompt union opposition. Labor has been the chief obstacle to overhauling California’s notorious Environmental Quality Act—a reform supported by Democrats and environmentalists—because unions like using the law’s excessive paperwork burdens to threaten projects important to employers.

Big labor is behind a New Jersey state senator’s proposal last week for a trillion-dollar federal bailout of state and local government pensions—pensions that most federal taxpayers who would be paying for the bailout can only dream about.

Big labor is behind $15 minimum-wage proposals in major cities—a high-risk experiment for low-skilled workers, who may find themselves without jobs. But it will be a winner for organized labor. Not only will it raise costs for nonunion businesses. In Los Angeles, unions seek their own exemption so they can conspire with employers to substitute untaxed benefits for taxable wages, which strengthens the union’s hold on workers while shifting costs to other taxpayers.

As Miles Kimball, a University of Michigan economist who calls himself a “supply-side liberal,” wrote on his blog a couple of years ago: “Most unions are middle-class organizations that in their political activities are ready and willing to sacrifice the interests of the poor to benefit their members and their leaders.”

Mr. Walker’s revolution was driven by voters in towns and small cities who noticed that government workers had morphed into a privileged class—with the best pay, best benefits, longest vacations, and job security that made them basically unremovable.

Their dues, meanwhile, funded a political class that seemed indifferent to anyone else’s problems. Not always a friendly source, the Milwaukee Journal-Sentinel this year credited Mr. Walker with having “shifted the policies of his state more than anyone else in generations.”

A heavy-breathing Mother Jones commentary at the time accused Mr. Walker of engaging in a “deeper game” to deprive Democrats of union funding and infrastructure that “allows the opposing party to exist at all.” The tiny smidgen of truth here actually shows how misleading such partisan simplicities are. Ted Kennedy (because he was Ted Kennedy) could buck the unions and promote airline deregulation in the 1970s. He bucked the unions in the early 2000s to push the school-accountability law No Child Left Behind.

But even Kennedy said his greatest legislative regret was letting labor block a national catastrophic health-insurance compromise with the Nixon administration in 1974.

Bill Clinton, Dick Gephardt and Tom Daschle were keen to back private Social Security accounts in the 1990s—until labor shut them down. More than 20% of delegates at Al Gore’s 2000 convention were union members—and said their goal was to make sure the platform said nothing about entitlement reform.

Mr. Walker recently and unnecessarily burnished his antiunion credentials by signing a bill he’d previously resisted on right to work—which, contrary to label, actually deprives employers of their freedom of contract (e.g., Harley-Davidson, a Wisconsin employer that finds value in its union relationships).

His important fight, which led to the 2012 recall effort, unwound a 1959 experiment in union representation for public employees that proved a bad idea for all the reasons FDR and other traditional liberals warned. It gave rise to what some call the blue-state governance model, combining a stagnant, overtaxed private sector, a bloated public sector, and a long-term pension time bomb of the sort nowadays blowing up in cities and states (and Puerto Rico) around the country.

The beef against Mr. Walker is a lack of breadth—a common snipe when it comes to inland governors—and an alleged reluctance to take advice. But paragons of presidential virtue are overrated when what’s needed is a candidate who can do the job that needs doing now. And he enters Thursday’s debate as the No. 2 runner in a Fox News Poll, just behind Donald Trump.

His real failing is that he belongs to the wrong party. An antiunion Republican candidate is practically a redundancy these days. Yet even a GOP president backed by a GOP Congress won’t be able to carry forward meaningful reforms unless Democratic reformers are willing to step up too.

To change America’s path will require both parties. Too bad we don’t have a Democratic Scott Walker yet. We’ll need one.

Certainly the states run by public-employee unions, particularly teacher unions, need someone willing to stand up to the most malignant force in politics today.