For seven years, nothing united U.S. Republicans like their shared hate-on for Obamacare. In unison, they cried "repeal" at the very mention of former president Barack Obama's 2010 health-care law and the subsidies it extended to millions of uninsured Americans to buy coverage.

So, why is the opportunity to finally kill the law now threatening to blow up the GOP?

Republicans have discovered that repealing and replacing Obamacare is only slightly less complicated than brain surgery. In its short life, Obamacare has transformed vast swaths of the U.S. health-care industry and added more than 20 million Americans to the insurance rolls. Beyond the newly insured, it has created powerful vested interests dependent on its survival.

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That does not mean it has been a success. The chief executive officer of Aetna, the massive U.S.-managed care provider, last month became the latest critic to declare that Obamacare is in a "death spiral." Despite generous subsidies, not enough healthy Americans have opted to buy insurance on Obamacare exchanges, leaving insurers with a pool of disproportionately sick customers. Unable to recover their costs, several large insurance companies have pulled out of the exchanges or plan to do so. Hence, the "death spiral" many say will ultimately bury the program.

Watching this unfold has made many Republicans leery about taking ownership for health-care reform. Break it and it's yours. Rather than tinkering with a fundamentally flawed program – and getting blamed by voters if they botch it – some Republicans see no gain to be had in replacing Obamacare with a GOP version that would only face many of the same contradictions.

Besides, the hard right of the party is against state involvement in the health market, period. Libertarian senators such as Mike Lee of Utah, Rand Paul of Kentucky and Ted Cruz of Texas favour a clean repeal of the Obamacare law – with no replacement. These advocates of "personal responsibility" are echoed, more crassly, by the likes of Kansas congressman Roger Marshall, who declared that poor Americans "just don't want health care and aren't going to take care of themselves."

Thankfully, U.S. President Donald Trump has a heart after all, at least when it comes to health care. "We're going to have insurance for everybody," he said in January. "There was a philosophy in some circles that if you can't pay for it, you don't get it. That's not going to happen with us." The problem is, upholding that vow means that Trumpcare must inevitably look a lot like Obamacare.

It starts with preserving Obamacare's core features – prohibiting companies from denying insurance to those with pre-existing conditions or setting a lifetime dollar limit on coverage.

The Trump-backed health-care bill House of Representatives Speaker Paul Ryan has introduced would replace Obamacare's income-based subsides to purchase insurance with less generous fixed-tax credits based on age. Though it would repeal the Obamacare mandate, which forces all Americans to hold health insurance or face a tax penalty, it would allow insurers to sharply raise premiums on customers who let their coverage lapse and purchase insurance only when they need it.

While it would eliminate most Obamacare taxes, the Ryan bill would continue until 2020 to fund states' expansion of their Medicaid rolls that began under Obamacare. Medicaid is a joint federal-state program that provides insurance to the very poor. But to rein in Medicaid costs, the Ryan bill would replace Obamacare's open-ended funding with federal per-capita grants to the states.

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To the rogue Republicans opposing his bill as Obamacare Lite, Mr. Ryan last week responded: "This is the closest we will ever get to repealing and replacing Obamacare." He's right about that. But his bill has plenty of hurdles to clear before it crosses the finish line, not the least of which are moderate Republican senators from Ohio, Alaska and Maine who say the Trumpcare tax credits aren't generous enough and the Medicaid rollback is too harsh.

Therein lies Mr. Ryan's challenge. He needs every Republican (or almost) to vote for a bill that many currently oppose as either too liberal or too conservative. Lobby groups on both sides – from the U.S. Chamber of Commerce (for) to the Koch-brothers-backed Americans for Prosperity (against) – will likely spend millions on ads to sway public opinion.

Even if it passes, Trumpcare will face many of the same fundamental design flaws facing Obamacare while covering fewer Americans. And Republicans risk feeling sick on midterm election day in 2018.