“OBAMACARE IS dead next month if it doesn’t get that money,” President Trump told the Wall Street Journal on Wednesday in a barely veiled threat to defund a crucial part of the Affordable Care Act. The president delivered this threat even though he has no viable replacement plan. Senate Minority Leader Charles E. Schumer (D-N.Y.), the president said, “should be calling me and begging me to help him save Obamacare.”

No. Mr. Trump should be working to preserve the Affordable Care Act, which is delivering health insurance to millions of Americans.

The president’s comments came after he reanimated the drive to repeal and replace Obamacare on Fox Business earlier Wednesday: “We have to do health care first to pick up additional money so that we get great tax reform,” he said. “So we’re going to have a phenomenal tax reform, but I have to do health care first.”

More desperate than clever, Mr. Trump’s talk of annihilating Obamacare, for which he would be justly blamed, is unlikely to coerce Democrats into supporting anything like the House Republican repeal-and-replace plan he backed, which failed to attract enough GOP support to pass the House. The indecency of Mr. Trump taking millions of Americans’ health care hostage is compounded by his suggestion that repeal-and-replace is about freeing budgetary space for Republicans to tinker with the tax code rather than about fixing health care. Even posing his threat, meanwhile, is astonishingly reckless.

After House Speaker Paul D. Ryan (R-Wis.) dropped a repeal-and-replace bill favored by Mr. Trump and GOP congressional leadership, the quixotic effort to end Obamacare seemed to be over. “We’re going to be living with Obamacare for the foreseeable future,” Mr. Ryan admitted. With reform stalled, Republicans now in control of the government had no responsible choice but to administer a system that has provided health coverage to millions and requires competent stewardship.

(Peter Stevenson/The Washington Post)

But, weeks later, it remains unclear whether Republicans will sustain the law or sabotage it. Insurance companies are terrified that Republicans will pull money from subsidies that help Americans pay out-of-pocket health-care costs, subsidies that help keep health-care markets stable. Without them, insurers would have to hike individual market premiums by some 19 percent, likely leading to the market meltdown that Republicans have warned against.

Continuing these payments is only the first and most obvious step Mr. Trump must take to shore up the health-care system. The president should continue fighting a lawsuit charging that spending on these cost-sharing subsidies is illegal absent further congressional appropriations, and he should press lawmakers to make those appropriations. He should also direct his administration to enforce Obamacare’s individual mandate, which requires Americans to obtain health-care coverage, so long as the system depends on the mandate to keep markets viable.

Instead, the Trump administration has cultivated uncertainty, giving up on repeal-and-replace then putting it back on the agenda, signaling that it would continue funding Obamacare subsidies then threatening to end them. In a health-care system that relies on cooperation between private insurance companies and the federal government, uncertainty can be fatal. Insurers must soon decide whether to stay in Obamacare markets and how much to charge if they do. They cannot like what they are hearing.