Joe Biden has come out of the South Carolina primary as the most plausible moderate candidate. Now is the time for Democrats to coalesce around him and make Super Tuesday a terrible day for Bernie Sanders.

Plenty of polling shows that Sanders would perform worse than other Democrats with swing voters in November even before President Trump dumps hundreds of millions of dollars of negative ads on his head. But I readily concede that I could be wrong about his election prospects. I misjudged Trump’s electability in 2016, and I might be misjudging Sanders’s today. I was right, however, that Trump would turn out to be an awful president — and I am convinced that Sanders would be awful, too.

Part of the problem is Sanders’s hectoring personality. He is a closed-minded ideologue who shows little willingness to compromise and little ability to bring people together. He prefers denouncing those who disagree with him as sellouts rather than trying to persuade them. That helps to explain why in 29 years in Congress he has been the lead sponsor of only three bills that became law — and two of those were to name post offices.

From my perspective, Sanders’s inability to get results is actually a blessing because his agenda is so extreme, but he could easily borrow Trump’s tactic of trying to enact his proposals by executive order. Sanders calls his agenda “democratic socialism.” It can be summed up as: Free stuff for everyone!

Sanders’s signature issue is Medicare-for-all: He would abolish private health insurance and provide everyone a federal plan that would cover just about everything (including dental, hearing, vision, prescription drugs and long-term care) without any premiums, deductibles or co-pays. This is far more generous than other wealthy countries, which typically have stingier state-provided health plans supplemented by private insurance.

But wait. Sanders is just getting started. He also promises to offer free public-college tuition for all; to cancel student debt and medical debt; provide high-speed Internet for all; build 10 million “affordable housing units”; expand Social Security; offer all families “free, high-quality child care and prekindergarten”; guarantee federally funded family and medical leave; rebuild the nation’s infrastructure; and provide a federal job to everyone who wants one.

As if that weren’t ambitious enough, Sanders advocates a Green New Deal. His goal is the complete decarbonization of transportation and electricity by 2030 and of the whole economy by 2050. He somehow proposes to achieve this hugely ambitious objective even while phasing out nuclear plants and natural-gas plants — the two main sources of low-emission power today. This makes no sense but is typical of Sanders’s radical, unrealistic, ruinous agenda.

Republican Tommy Blackwood of North Charleston, S.C. wants President Trump to stay in office, so he's trying to disrupt the Democratic primary race. (The Washington Post)

Sanders would double the size of the federal government, concentrating vast power in Washington while wiping out entire industries. (The health insurance industry alone serves 180 million people and employs 529,800 people — more than the entire steel industry.) According to CNN, Lawrence Summers, President Bill Clinton’s treasury secretary, estimates that Sanders’s plans “would increase government spending as a share of the economy far more than the New Deal under President Franklin Roosevelt [or] the Great Society under Lyndon Johnson.”

Until recently, Sanders was cavalier about how he would pay for the biggest expansion of government in peacetime U.S. history. He told “60 Minutes” in an interview that aired on Feb. 23: “I can’t rattle off to you every nickel and every dime.” Now, under pressure from his critics, he has released a plan to pay for his wish list. But it doesn’t add up. As Ronald Brownstein explains in the Atlantic, Sanders has identified about $30 trillion in new taxes and $12 billion in savings and other revenue. This would represent the biggest peacetime tax increase in U.S. history as a share of gross domestic product, but it would still fall far short of paying for up to $60 trillion in proposed spending over 10 years.

Sanders claims he will tax only “the rich,” but there’s no way he can get that much money out of “the billionaire class” he despises — especially because it has ample resources to fight the Internal Revenue Service. There are only two ways to fund his agenda: either massive tax hikes on the middle class or massive debt increases. Sanders never acknowledges either possibility, because either option could endanger our economic well-being.

Under Trump, publicly held federal debt has already hit almost $17 trillion, or 80 percent of GDP. The debt-to-GDP ratio is expected to climb to 144 percent by 2049. That’s already an unprecedented level of debt (higher than that of all but three countries today), and Sanders risks vastly increasing it. Talk about a bitter irony: While wiping out the existing student loans of today’s college graduates, Sanders likely would saddle them — and everyone else — with unsupportable levels of future debt.

Sanders claims to be a Scandinavian-style socialist, but the Scandinavian countries are far more fiscally responsible. He is really a Santa-style socialist offering goodies to everyone whether they’ve been naughty or nice. His generosity with other people’s money would risk a debt crisis and endanger our prosperity. Please, Democrats, don’t give him the chance to wreck either your election prospects or our economy.

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