It’s time to take Bernie Sanders seriously. The Vermont Senator is leading in Iowa and New Hampshire and in Sunday’s debate he sounded for the first time like a candidate who thinks he can win. He still isn’t the favorite against the Clinton machine, but it’s no longer impossible to imagine the 74-year-old socialist as the Democratic nominee.

It’s fashionable to describe Mr. Sanders as the progressive version of Donald Trump, but that is true only in that they both tap into public dissatisfaction with the status quo. Mr. Trump has populist instincts but few fixed ideological principles. Mr. Sanders is a true-blue man of the left. He would be the most left-wing Democratic nominee since George McGovern in 1972, but his economic agenda is more radical and hasn’t been seen in the U.S. since the 1930s.

This week he rolled out his Medicare-for-all plan that would end private health insurance. Mr. Sanders can see the public frustration with ObamaCare and wants to turn it to the left. The Vermont Senator also wants to make college and child care into universal entitlements paid for by the government, spend $1 trillion on public works, raise the minimum wage nationwide to $15 an hour (from $7.25) and greatly expand Social Security benefits.

Mr. Sanders would also use government to control the means of production, as the socialists used to say, starting with the banks. He would break up the largest via a “21st century Glass-Steagall” that suggests something far more ambitious than merely dividing investment from commercial banking. He says climate change is his main national-security concern, and he has called for a criminal investigation of climate dissenters.

His many tax increases would include a sharply higher rate for high earners that he doesn’t specify, though he does often speak fondly of the tax rates of the pre-Reagan (70%) and New Deal (94%) eras. He’d introduce a new payroll tax for the middle class to pay for universal health care and eliminate the income cap on the Social Security payroll tax (now $118,500) on earnings above $250,000.