The individual mandate is a crucial ingredient in the ACA’s policy design. Without it, many young and (at least for the moment) healthy people would skip buying health insurance, leaving only older and sicker people in the risk pool. That’s known as “adverse selection,” and absent the individual mandate, the ACA’s prohibitions on lifetime coverage caps and on refusing coverage to those with pre-existing conditions would make the problem catastrophically worse. But Senate Republicans have tacked an individual mandate repeal onto their tax bill because doing so gives them about $300 billion worth of additional room to cut taxes without blowing past a $1.5 trillion limit on the amount the bill would add to the national debt after a decade. Even if they don’t succeed, the integrity of the policy is threatened by the possibility that the Trump administration will simply stop enforcing the mandate. Already, it has weakened the law by accepting tax returns in which the question about health insurance coverage is left blank.