THE NEXT time someone argues that a businessman would manage the country better than an experienced politician, remember this past week. The attempt by President Trump and House Republicans to force through a health-care bill scorned by experts across the spectrum, projected to be a disaster for aging and low-income people and opposed by a large majority of Americans ended in debacle. Now the danger is that a wounded president and his GOP allies will act on their sore feelings by irresponsibly attacking the existing health-care system in other ways.

The right course for Mr. Trump and congressional Republicans following their decisive defeat would be to ensure that the system created by President Barack Obama is properly overseen, for the sake of the millions who depend on it. That would mean abandoning their unilateral and unpopular legislative push to replace Obamacare with a radically different scheme. None of the major repeal-and-replace proposals they have offered would improve the system — and repealing Obamacare without a replacement would invite disaster in health-care markets.

Unfortunately, there are signs that Mr. Trump will act rashly on his own, without Congress, weakening Obamacare on purpose or by sheer incompetence. Several times in recent weeks, Mr. Trump suggested that it would be savvier for Republicans to let the system persist — and collapse. Independent experts, including the Congressional Budget Office just this month, predict no such crumbling. Yet they may not have satisfactorily considered the likelihood of administrative sabotage: The Trump administration has already undermined federal enrollment efforts and the individual mandate that holds the system together. Health and Human Services Secretary Tom Price, who would lead any executive-branch regulatory overhaul, has shown himself to be a rigid ideologue on health-care policy.

Mr. Trump should not imagine that angry Americans will blame Democrats, who are totally locked out of power, if he presides over an unraveling of the system. Public reaction to the replacement effort, including in polls, showed substantial support for Obamacare and rejection of the Republican effort to destroy it.

A better option would be the one that Republicans have explored least: actually fixing the system’s flaws. Mr. Trump could use his executive power to shore it up — enhancing enforcement of the individual mandate and encouraging people to sign up. Then he should approach Democrats to see if there is room for an agreement on a repair bill. This would have to be an authentic deal, not an ultimatum, in which Democrats traded things Republicans want, such as medical liability reform and some limited regulatory reform, in exchange for things they should want, such as enhanced subsidies for vulnerable people.

For the good of the country, Republicans must finally admit two things. First, Obamacare is mostly working and millions will be hurt if it is abruptly repealed. Second, the GOP is incapable of the near-unanimity on health-care policy that is required to act without Democrats.