Republicans weren’t supposed to drive themselves into this particular box canyon.

“We want to make sure that people with preexisting conditions continue to get covered,” Rep. Greg Walden (R-Ore.) said in a January 2017 interview with CNN during the GOP issues retreat in Philadelphia that charted the agenda.

As chairman of the House Energy and Commerce Committee, Walden was charged with writing legislation intended to finally repeal Obamacare. Walden, coming off a four-year stint as the top campaign strategist, knew one thing for certain: Do not mess with the Affordable Care Act’s provision forbidding insurers from denying coverage because of preexisting medical conditions such as cancer, asthma or diabetes.

So the first draft of the American Health Care Act left that very popular provision intact and House Speaker Paul D. Ryan (R-Wis.) advanced the bill to the floor in late March — only to run into a conservative rebellion led by Rep. Mark Meadows (R-N.C.) and the House Freedom Caucus.

After a hasty meeting with President Trump, Ryan pulled the bill from the floor.

Looking back with 20/20 political hindsight, those next six weeks became the most critical period of this midterm election season. Ryan and Trump caved to the conservatives and inserted language in the bill that weakened the preexisting condition rules — allowing states to let insurers charge some people more — and thereby handed Democrats their most lethal political weapon against Republicans.

Democrats are now slightly favored to win the House in Tuesday’s midterm elections, an outcome that might have happened even if Republicans had never added the provision to change the 2010 law’s core protection. The original health-care proposal already faced a hostile audience with estimates showing more than 20 million fewer people would have insurance over the next decade.

Giving in to the conservative demands on preexisting conditions provided the final votes to approve a new version of the repeal bill in early May, prompting singing chants of “Na Na Hey Hey Kiss Him Goodbye” from Democrats.

Now, more than any other issue, preexisting conditions have dominated the airwaves in House and Senate races. Almost 55 percent of all Democratic ads from mid-September to mid-October mention health care, broadly speaking, according to a study by the Wesleyan Media Project, and almost every single one mentions the threat to preexisting conditions.

Republicans are just beginning the recriminations over that decision 18 months ago, particularly among those now facing the barrage of ads focusing on that move.

“The members who were calling for those changes really don’t care. And they’re very selfish, they always have been,” Rep. Carlos Curbelo (R-Fla.), who voted for the bill, said in an interview last week outside his campaign headquarters in Miami.

Meadows, through a spokesman, declined to comment about the internal GOP debate in early 2017.

The fallout blasted far and wide, well beyond the few dozen members of the moderate Tuesday Group caucus who had been supportive of Walden’s original stance. Prominent centrist Reps. Tom MacArthur (R-N.J.) and Fred Upton (R-Mich.), who negotiated the final details of the language on preexisting conditions, now find themselves in the fight of their political lives, under constant attack for their work with Meadows.

Meadows’s own Freedom Caucus colleagues, particularly Reps. Dave Brat (R-Va.) and Scott Perry (R-Pa.), find themselves in toss-up races and on the defensive about that May 2017 vote.

Perhaps worst of all, the Senate choked on its own ACA repeal bill and Obamacare is still the law of the land. House Republicans walked a political plank that got sawed off behind them, leading to false hope that the issue would fade away by Tuesday’s election.

A group of Republican state attorneys general filed a lawsuit questioning the health law’s constitutionality, and the Trump administration sided with them, keeping the issue front and center.

Republicans now sound exasperated about the issue. “It was only a two-page amendment,” MacArthur said on a recent telephone town hall with seniors in his central New Jersey district.

Politically, it was so much more than that. “The original sin of the Republican Party was that health-care vote, and nothing was more sinful than gutting that preexisting condition guarantee,” said Jesse Ferguson, a Democratic strategist who consults for liberal groups running health-care ads.

An August survey, by Kaiser Health, found that 75 percent of Americans supporting the ACA’s guarantee for preexisting conditions.

But, in late March 2017, the Meadows wing argued that eliminating mandates on insurance companies would lead to lower premiums for consumers.

After the initial failure, the North Carolinian sat down with MacArthur and came up with a complicated process that allowed states to grant waivers to insurance companies so they could opt out of guarantees for preexisting conditions and some essential health benefits.

Some Republicans believed this was such a high hurdle that few states would have actually tried to get those waivers. Others believed that the impact would be slight — very few actually denied coverage because of preexisting conditions and limited impact on premiums.

If that was the case, some Republicans privately wondered, why go through with such an effort? All they did was open themselves up to Democratic lines of attack.

By late April 2017, then-Rep. Charlie Dent (R-Pa.), a leading moderate, accused Meadows of just going through “an exercise in blame shifting,” so that if the repeal effort died again it would be the Tuesday Group that had sunk it.

Advisers to the conservatives bristle at the notion that they forced anyone to vote for anything, that each lawmaker can exercise his or her own vote.

Ultimately, most Republicans decided that voting for the new repeal bill was more important than the risk associated with denying insurance guarantees.

“I think honesty and sincerity are the most important characteristics or qualities for anyone in public office,” Curbelo said. “And I made a commitment when I first ran, in 2013, that I would try to repeal the ACA and replace it with something better.”

In February 2017, Walden introduced a stand-alone bill to protect guarantees for preexisting conditions. That legislation never advanced beyond a subcommittee, but the list of its 81 co-sponsors reads like a Who’s Who of endangered House Republicans trying to prove their bona fides on the issue.

The final GOP lawmakers signed on in the late summer as the political climate became more dire, including one familiar name: MacArthur.