WHEN OBAMACARE repeal-and-replace stalled in the House, GOP lawmakers revived it by eroding regulations protecting vulnerable people, thereby attracting support from the far right of the Republican caucus. Conservatives are now angling to do the same in the Senate.

Facing opposition from both moderates and right-wingers, Majority Leader Mitch McConnell (R-Ky.) put off a vote on the Senate version of repeal-and-replace, which he had hoped to take up before the July 4 break. Now, as Mr. McConnell woos moderates with more spending on treatment for opioid addiction, conservatives are demanding more aggressive deregulation of the insurance industry, floating ideas that would unravel the Affordable Care Act’s carefully designed markets for individual health insurance.

Sen. Ted Cruz (R-Tex.) proposed allowing insurance companies to sell plans that do not comply with Obamacare mandates, which require them to serve people with preexisting conditions and cover a set of essential health-care services. The companies would still be required to sell at least one compliant plan, too, but that should be little comfort to people with health conditions, older people and others at risk of insurance company discrimination.

Instead of grouping many people with varying health-care needs together, spreading costs so that no one pays an insupportable amount, Mr. Cruz’s plan would create a two-tiered system. Insurance companies would sell cheaper, noncompliant plans to healthy, low-risk customers. Sicker and older people would be left in the ACA-compliant market, where costs and premiums would soar. Only people with expensive medical needs would shell out for coverage, insurance companies would have to boost premiums to cover their costs, and more customers would exit the market. As the pool became ever-sicker, premiums would go ever-higher.

In theory, federal subsidies could protect vulnerable people from cost spikes, at least for a time. But the subsidies would get skimpier under the Senate plan, and many people would not qualify for them. Absent a massive infusion of money, other funds in the bill meant to stabilize premiums could only do so much to hold back the spiral, and they would eventually expire anyway. People with serious medical needs would surely get hurt.

Equally destructive would be Sen. Ben Sasse’s (R-Neb.) proposal to repeal Obamacare without simultaneously replacing it. After passing a bill ending Obamacare at a certain date, the thinking goes, Congress would have more incentive to enact a conservative replacement. The GOP seemed to have abandoned this idea earlier this year, yet it received an unexpected boost from President Trump, who tweeted favorably about the strategy late last month.

Mr. Trump has repeatedly threatened to tank Obamacare markets to compel Congress to pass a health bill. That may not be what Mr. Sasse has in mind, but it would be the effect of his approach. Absent a clear replacement and transition plan, industry and policy experts warn that insurers would quickly abandon Obamacare markets slated to be eliminated, stranding customers who currently depend on them. A spokesman for Mr. Sasse argues that Obamacare markets “are already in collapse,” a twisted take on reality that gets no more convincing the more Republicans repeat it. Even if it were so, Republicans would bear much of the blame for mismanaging the system.

Neither of these ideas deserves a hearing. Instead of continuing with a repeal-and-replace effort that conservatives are pulling in a disastrous direction, moderate Republicans should finally work with Democrats in good faith to improve Obamacare.