When GOP lawmakers pushed their tax bill through Congress late last year, they made sure to include a long-standing item on their wish list: an end to the Affordable Care Act’s “individual mandate” penalty, which charged tax filers a fee for not having health insurance.

The penalty was part of the ACA’s “three-legged stool.” The law forced insurers to meet coverage minimums and prohibited them from discriminating on the basis of preexisting conditions, opening up the individual insurance market to millions who had been all but barred from it. The bill compelled every American to sign up for coverage, thereby creating a pool of both sick and healthy people and spreading risk among as large a population as possible. And it set up subsidies to defray premium costs for low- and middle-income enrollees, making the mandate less burdensome.

Many Democrats worried that killing off the mandate penalty would undermine Obamacare, allowing too many healthy people to leave the market, thereby making the insurance pool more expensive to cover. The policy architect of Obamacare penned an op-ed in Fortune portraying the mandate as a linchpin of the law.