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Two months ago, Elissa Slotkin ran an advertisement that came to define not only her campaign, for a rural Michigan district, but the entire Democratic Party case to depose the Republican majority. The ad began with Slotkin talking about her mother’s first fight to survive cancer, which left her with a preexisting condition. Then the ad zoomed in on video of the incumbent Republican congressman, Mike Bishop, smiling and receiving a handshake from President Trump in the Rose Garden after voting to repeal Obamacare in 2017.

Bishop’s district was gerrymandered by his party to ensure Republican control, and supported Donald Trump by almost seven points in 2016. FiveThirtyEight calculated the race between Slotkin and Bishop was the likeliest tipping-point district that would determine control of the House. Slotkin has knocked off Bishop, and her victory was more than just a bellwether. It was the thematic heart of the Democratic campaign to win back control of the House.

No single factor explains any election. First-term presidents almost always face a backlash from a ginned-up opposition, and hardly a day has passed over the last two years in which Trump has not done something to gin up his opposition. But for the almost infinite list of reasons voters broke the Republican hammerlock on the House, health care has been the dominant theme. It is their most visceral and undeniable betrayal of their promises to the voters. Attempting to repeal Obamacare probably cost Republicans the House.

Health care has registered as the highest voter priority in exit polls. It also featured more heavily in Democratic advertising than any other issue. As the Republican consultant Rick Wilson explained in early 2017, Republicans meeting with focus groups could not produce any argument that would bring the public around to their fundamental view, which called for repealing the law’s signature regulation of the insurance market. “We were prepping anti-Obamacare ads. In EVERY group and I mean EVERY group,” he recalled, “Democrats, indys, hard Rs, soft Rs, rich, poor, black, white, urban, suburban … there was one argument that nuked everything else … and that was coverage for preexisting conditions. It didn’t matter where you were coming from on the battle … it was the killer app.”

What Paul Ryan’s party did to itself on health care has almost no parallel in history. What happens when a party votes for a high-profile bill that polls at 17 percent nationally? We just found out.

The more fascinating question is why Republicans marched themselves into this political deathtrap. The most likely answer is that they came to believe their own propaganda.

Since 2009, the Republican Party’s posture on health care has been a giant lie. Republican elected officials promised in public to work cooperatively to solve the problems in the system, especially a broken individual market in which insurance was unaffordable to anybody with a preexisting condition. But both their political interest in denying Obama a victory and their ideological opposition to bigger government in any form drove them to a very different goal: blocking any bill at all. Republicans strung out negotiations with the Obama administration for a year, publicly blaming Democrats for rushing through a partisan bill while refusing to be pinned down on any concrete proposals or demands.

They insisted if Obamacare could be blocked — and then, after it passed, repealed — Republicans would design a better alternative that would do all the good things Obamacare delivered, without costing anybody anything. Supporters of Obamacare believed the absurdity of the lie could not be sustained, that the successful operation of the law would create beneficiaries who would demand its protection.

And yet it seemed for years on end that the lie would never be exposed. Republicans could sabotage the law at the state level, and Democrats would be blamed for its shortcomings. In places where Obamacare was helping people, its beneficiaries largely did not realize it. A stream of tragicomic reporting revealed people whose lives had been changed for the better by Obamacare believing they were not on the program at all, or voting for Trump out of the conviction that he would preserve or even expand the program. In this upside-down world, Republicans could work feverishly to take access to health care away from millions of Americans while piously promising to deliver it to one and all — and being believed.

Two years ago, I wrote a column conceding that, while the law had worked about as well as intended, on a purely political level it had failed. The law’s supporters “assumed that perception would eventually catch up with reality,” I wrote, “but this has not happened, and there is little reason to believe it will anytime soon.”

But this, too, turned out to be wrong. And the reason was that Republicans experienced the catastrophic success of winning an election and being expected to carry out their impossible promises.

Having spent hours watching Fox News, Trump had imbibed the party’s position and boiled it down to its essence: “You’re going to have such great health care at a tiny fraction of the cost,” he promised, “and it’s gonna be so easy.” This was a straightforward extrapolation of years of conservative messaging that Obamacare was an unworkable train wreck laden down with costly and unnecessary bureaucracy. Trump almost certainly did not realize that Fox News had been fooling him along with all the other conservatives. Probably most Republicans believed the nonsense too: that Obamacare was a train wreck, that nobody would miss it, and that Republicans could easily write an alternative that people would like.

When they had to deliver, the inherent trade-offs of health-care policy, in which everybody’s coverage needed to be paid for somehow, in combination with the immovable ideological rigidity of conservative-movement anti-government dogma, produced the only possible outcome. The Republican bills would have inflicted medical deprivation on millions. Their bill was historically unpopular. It had the extraordinary effect of making Obamacare actually popular for the first time. Obamacare has been the killing field for the eight-year-long Republican House majority.

The irony is that Republicans had so many chances to avert this fate. They could have compromised with Obama in 2010, rather than enforce a party-wide boycott on legislative compromise. They could have let the matter drop after the bill passed. They could have compromised again after 2016, working with Democrats to patch up the bill’s flaws rather than repealing its core coverage guarantees. They pressed forward because they believed their own lies. After nine years, the political world has turned right-side up.