President Donald Trump’s first reaction to the implosion of Republican efforts to repeal and replace the Affordable Care Act was to blame Democrats—never mind that he never asked for their help, or that the process was designed explicitly so that no Democrats’ votes were needed.His second reaction, however, was more troubling: He warned that “Obamacare will explode,” expressing his desire to let the system set up under the health-care law to collapse of its own accord.It’s a threat we should take seriously.