The Supreme Court saved Obamacare. Now supporters want Biden to fix the law.

After the latest Supreme Court ruling on Obamacare secured its survival, some of the law’s staunchest supporters have a clear message on what President Joe Biden should do next: Fix it.

The Affordable Care Act, after more than a decade of political turmoil, has never loomed larger. The law provided a new safety net during the coronavirus pandemic, mostly through its expansion of Medicaid. The Biden administration has boosted federal aid to purchase Obamacare coverage, which could help bring in millions of new customers. Insurers who fled the marketplaces in the law’s turbulent early years have returned,...