It’s not very complicated, or even very subtle. The left’s disagreements with Wisconsin Gov. Scott Walker have nothing to do with evolution, or whether President Barack Obama is a Christian — though some in the media have sought out Walker’s opinions on those matters in recent weeks.

It’s all about unions. The Washington Post acknowledges as much in a Monday report about Wisconsin’s version of a right-to-work law.

“The anti-union law passed here four years ago, which made Gov. Scott Walker a national Republican star and a possible presidential candidate, has turned out to be even more transformative than many had predicted,” the Post explained. “Walker had vowed that union power would shrink, workers would be judged on their merits, and local governments would save money. Unions had warned that workers would lose benefits and be forced to take on second jobs or find new careers.”

What, exactly, was in those reforms?

First, there were measures to require union workers to pay something toward their retirement and health care plans — something all other workers in the country already do.

“For years in the state system, state government and municipalities paid not only the employer share of pension contributions but — primarily due to collective bargaining — essentially all of the employee contributions,” reports the Milwaukee Journal-Sentinel’s Politifact project. “Act 10 requires that for most public employees, pension contributions be split equally between employees and employers.”

The law also required union members to pay at least 12 percent of their health care premiums.

The result, as Politifact acknowledges, is that the state has saved $3 billion.

The reform measures also included a right-to-work provision — meaning that employees are no longer forced to be in unions just to hold jobs. And it limited the power of unions to negotiate raises and benefits for their members, above a certain level, without a public referendum.

That’s what has the left upset. What they’ve found is that when people aren’t forced to pay those union dues, they don’t.

The Washington Post’s piece on Walker includes interviews with former union members. The interviews are very revealing.

“I don’t see the point of being in a union anymore,” said one teacher. “Everyone’s on their own island now. If you do a good job, everything will take care of itself. The money I’d spend on dues is way more valuable to buy groceries for my family.”

Another teacher added that unions are “just not something I concern myself with… I just look to keep improving my teaching in the best way I can and try to keep my nose out of the other stuff,”

This is important because unions remain a core constituency of the Democratic Party’s base. And this is why Walker is now in the cross-hairs.

As the Post’s Jennifer Rubin observes, “the Democratic Party and Big Union allies tried to recall him when he took on public employee unions; he triumphed. They made him a target in 2014; he again won handily.”

So it’s not really about Obama, or Rudy Giuliani. It’s all about the base.