Imagine you’re president and Congress gives you a huge chunk of money to spend as you wish. Instead of cutting taxes or splashing out more on health care, infrastructure and defense, why not send a check to every adult?

That’s the essence of universal basic income, a centuries-old idea now enjoying a revival across the political spectrum. Liberals see a cure for poverty that will be essential as robots and artificial intelligence vaporize jobs. Conservatives see an elegant, cost-effective replacement for a complex safety net...