Election Day is a reckoning for Republicans on Obamacare repeal.

On the issues, health care is rivaled only by immigration in the 2018 midterms — and, in a remarkable reversal from 2010, Democrats have been on the offensive. They’ve attacked Republicans again and again for the GOP’s various health care bills, which would have rolled back Obamacare’s protections for preexisting conditions.

The law’s insurance reforms have been the top-billed issue in many of 2018’s most critical races, but health care is on the ballot in a whole lot of ways: Medicaid expansion, prescription drug prices, Medicare-for-all, and women’s health are all at stake on Election Day.

But for veterans of 2017’s Obamacare repeal battle, two House races stick out from the rest: the New Jersey Third, where GOP Rep. Tom MacArthur looks to be in deep trouble, and the Michigan Sixth, where Republican Rep. Fred Upton is unexpectedly vulnerable.

MacArthur and Upton have the same problem: They didn’t just vote for Obamacare repeal, they personally intervened to help pass the bill in the House — and their efforts centered on the law’s protections for preexisting conditions.

If these House Republican incumbents go down, there can be no doubt about why — and they’ll have helped set the stage for their own downfall. Let’s quickly run through 2018’s marquee Obamacare repeal races.

New Jersey’s Third: Tom MacArthur’s bill comes due on preexisting conditions

What’s the district? The New Jersey Third Congressional District, representing the Garden State’s Philadelphia suburbs. The district voted for Donald Trump by 6 points in 2016. Cook Political Report rates it a toss-up.

Who is the Republican? Incumbent Rep. Tom MacArthur, who was first elected in 2014 and won reelection by 20 points in 2016.

Who is the Democrat? Andy Kim, a former White House counterterrorism official under President Barack Obama. He announced his candidacy shortly after the House passed its Obamacare repeal bill in spring 2017.

How Obamacare repeal defines the campaign: MacArthur is the prototypical Republican paying the price for the GOP’s plans to roll back preexisting conditions.

The New Jersey Republican didn’t just vote for the House GOP’s health care bill. He was instrumental in forging a compromise with the archconservative Freedom Caucus to loosen some of the Affordable Care Act’s protections, like essential health benefits, in a way the far right and the more moderate members of the caucus could agree on.

The part of the House bill that gave the states the option to unwind the law’s insurance reforms was literally named the MacArthur amendment. MacArthur was a forceful defender of the bill and his provision, even standing for hours at a town hall to take questions from constituents. He actually left his leadership position in the moderate Tuesday Group over the issue, in a sign of how far out on a limb he went for Obamacare repeal.

Kim directly tied his campaign announcement to MacArthur’s role in trying to repeal Obamacare. His campaign has repeatedly attacked MacArthur because he “wrote a bill to gut protections for people with preexisting conditions.”

MacArthur, like many other Republicans in the midterms, has tried to claim he supports protecting preexisting conditions despite this history. PolitiFact rated the GOP claim that the House bill, including the MacArthur amendment, kept those protections as “mostly false.”

Michigan’s Sixth: Fred Upton’s little-remembered role in Obamacare repeal put him at risk

What’s the district? The Michigan Sixth Congressional District, representing the state’s southwest corner. The district voted for Donald Trump by 8 points in 2016. Cook Political Report recently slid the race from Likely Republican to Lean Republican.

Who is the Republican? Incumbent Rep. Fred Upton, the former House Energy and Commerce chair, who was first elected to Congress all the way back in 1986. He won his last reelection by more than 20 points in 2016.

Who is the Democrat? Matt Longjohn (great or terrible name, I can’t decide), who was formerly the YMCA’s national medical director.

How Obamacare repeal defines the campaign: You might have forgotten about the Upton amendment, which was cooked up in the frantic final days before the House passed its Obamacare repeal bill.

But I remember, and so do Democrats. It was crucial to helping get the House bill across the finish line. Now Democrats are using it against Upton in pursuit of what would be one of 2018’s most unlikely House upsets.

After MacArthur cut a deal with the Freedom Caucus on rolling back Obamacare’s insurance regulations, moderates needed something to feel better voting for a bill that undermined preexisting conditions. Fred Upton found it.

Upton publicly came out against the bill two days before the eventual vote, citing concerns that it would erode protections for people with preexisting conditions. His defection was a stunner, given his experience as a health care legislator, and it was a blow to House leaders the same week they were supposed to vote.

But even as he announced his opposition, Upton was already working on a bridge back to “yes”: an additional $8 billion to help people adversely affected by the bill through the waivers states could obtain from Obamacare’s rules protecting people with high medical costs.

Experts said it wasn’t enough money to ensure those people didn’t see their costs increase, and Upton himself admitted he didn’t know if it was. No matter. When leadership and the White House agreed to his amendment, he was back in the fold.

Upton’s move was a rapid defection-then-conversion. The Congress member let slip in a scrum with reporters that he had actually started working on his fix before he came out against the bill. That raised a question: Why did he publicly come out against it at all?

Upton pulled out his cellphone to show reporters text messages, promising that his timeline made sense. But even some Republican staffers thought the whole ordeal seemed awfully convenient.

Never mind the dizzying policy mechanics, this new policy being the solution to a problem created by the initial concession to the Freedom Caucus. Never mind that moderates had earlier said states wouldn’t want these waivers anyway, but the Upton amendment actually incentivized states to pursue them because it set aside this new pot of money.

People would vote for it. That’s what mattered.

“My sense is that there are a number of folks that will come on board now that we have this fix,” Upton told reporters before the final vote. “I think there will be a number of ‘no’ votes before ... that will be okay with this bill.”

He was right. The Upton amendment brought three other moderate Republicans onboard with the repeal bill, and it passed.

But now Democrats want to make Upton pay. Left-leaning groups have gone on the air attacking the GOP incumbent for “leading the charge” on rolling back protections for preexisting conditions.

Upton should be considered the favorite heading into Election Day. He’s represented this district for 25 years. He’s outspent Longjohn throughout the year.

But if he falls, he’ll only have himself to blame. Health care has defined the 2018 campaign, and Upton put himself in that crossfire.

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Correction: This post has been updated to clarify that Fred Upton was first elected to Congress in 1986. He initially represented the Michigan Fourth, before being elected to the Michigan Sixth in 1992 after redistricting.