The Democratic presidential candidates are promising free everything: Free health care, free college, even free money for people who don’t want to work.

And to pay for all that free stuff, some candidates support raising the highest tax rates to 70% or higher.

But you gotta’ hand it to New York City Mayor Bill De Blasio. He’s come up with a whole new idea to rake in revenue — Tax robots.

“As President, I would issue a robot tax for corporations displacing humans, and create a federal agency to oversee automation,” De Blasio wrote in a Wired op-ed. “My proposal would institute a ‘robot tax’ on large companies that eliminate jobs through increased automation and fail to provide adequate replacement jobs. They’d be required to pay five years of payroll taxes up front for each employee eliminated. That revenue would go right into a new generation of labor-intensive, high-employment infrastructure projects and new jobs in areas such as health care and green energy that would provide new employment. Displaced workers would be guaranteed new jobs created in these fields at comparable salaries. …

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“As mayor of New York, and as a candidate for President focused on the needs of working people, I’ve seen at home and across the country that workers can benefit from these technological changes, but we can’t let American jobs be replaced by them,” he wrote. “That’s why I’m proposing a new plan today to protect American workers and ensure that we all share in the gains from technological advancement.”

De Blasio said his plan rejects a “post-work future.”

“Instead, it would hasten a work-filled future — one where we use technological advancement to bridge the gap between our current workforce at risk of being made redundant by automation and the resilient union workforce we need. A ‘robot tax’ will help us create stable, good-paying middle-class jobs for generations to come,” he wrote.

Grover Norquist, president of Americans for Tax Reform, blasted the proposal.

“It’s official. The Democrats want to tax everything,” he said. “De Blasio had to find a way to stand out after the rest of the Democrat field had already laid claim to raising taxes on income, payroll, carbon dioxide, small businesses, corporations, your 401k, lifetime savings, and death.”