If President Donald Trump and Republicans in Congress want to undermine the Affordable Care Act, also known as Obamacare, they’re going to be wreaking havoc in the places their voters call home.

In counties with just one or two insurers offering coverage through the law’s new exchanges, 54.5 percent of residents voted for Trump in 2016, while just 40.8 percent voted for Clinton, according to a Huffington Post analysis of election returns and data from the Henry J. Kaiser Family Foundation.

Overall, among the roughly 62 million voters in those counties, 34 million cast ballots for Trump while 25 million went for Clinton.

Only a small fraction of these people actually get their insurance through the Affordable Care Act ― and, among those, many are getting coverage through Medicaid, the joint federal-state program that the the 2010 health care law expanded.

But several million people are buying private insurance on their own, either directly from insurers or through the exchanges. These markets for people buying coverage on their own are where the Affordable Care Act has made its most dramatic changes ― by prohibiting insurers from denying coverage to people with pre-existing conditions, for example.

Some markets are working just fine, with insurers covering costs and even expanding. But for others, insurers have struggled to make profits under these conditions and have responded by raising premiums or withdrawing altogether ― in many cases, leaving just one or two carriers offering policies this year.

Fewer insurers can mean fewer choices for consumers, and higher prices because there’s less competition among carriers. In the worst-case scenario, some areas could be left with no insurers at all. Already, several counties in Tennessee have no insurers lined up for 2018, although state officials say they hope to persuade one carrier to return.

Trump and the Republicans have cited rising premiums and fleeing insurers as a pretext for repealing the entire law.

What Trump Could Do Through Neglect Or Outright Sabotage

In what sounded like a veiled threat after GOP leaders gave up on their repeal legislation, Trump said Friday that he expected the Affordable Care Act to “explode.” He repeated that on Twitter the following day.

Trump doesn’t have the power to blow up the whole program. But he does have options that would allow him to do damage, and maybe even severe damage, to those shaky private insurance markets.

The most extreme step the administration could take would be to drop the appeal of a 2016 court ruling that, if allowed to stand, would cut off funds insurers use to reduce out-of-pocket costs to lower-income consumers. House Republicans, who filed the lawsuit, argue Congress never appropriated money for that purpose. The Obama administration appealed right after the decision.

If the Trump administration backed off the appeal, money would stop flowing to insurers unless Congress went ahead and appropriated it. And without those funds, insurers would have to jack up premiums or pull out of markets altogether.

This would be true in the places where Affordable Care Act markets are struggling ― and even in those places where the markets are functioning smoothly.

The Trump administration could also ease up on enforcing Obamacare’s individual mandate, the penalty for consumers who don’t get coverage when they could. The administration has already announced changes that could weaken enforcement incrementally.

One more option for the Trump administration would be to stop promoting new enrollment. The Obama administration worked hard at this, but in January, as this year’s open enrollment was ending, the Trump administration canceled some of the planned outreach efforts. Sign-ups, which had been running ahead of last year’s pace, fell behind in the last two weeks and left the 2017 total slightly lower than it had been in 2016.

These steps would probably have a much bigger impact on the weaker markets ― a group that probably corresponds, roughly, with the counties that today have just one or two insurers and a preponderance of Trump voters.

Why Pro-Trump Counties Are So Vulnerable

This has always been one of the great ironies of the GOP’s assault on the 2010 health care law, which has brought the number of Americans without insurance to a historic low but raised premiums for many who had coverage previously. Wrecking the newly reformed private insurance markets could hurt Trump voters disproportionately.

Had Republicans succeeded in passing their bill to repeal the Affordable Care Act, older and poorer voters buying coverage on their own would have faced some combination of significantly higher premiums and out-of-pocket costs. Many of these people would have ended up dropping coverage altogether. In the 2016 election, these people were more likely to have voted for Trump rather than for Democratic nominee Hillary Clinton, according to an analysis by Nate Cohn in The New York Times’ Upshot.

Alissa Scheller/The Huffington Post

The disproportionate share of Trump voters in counties with weak markets isn’t surprising.

Trump’s support is strongest in rural areas and in the South. Rural areas are always difficult for insurers, because the sparse population makes it hard to construct balanced risk pools and hospital monopolies make it hard to extract price discounts.

The South, meanwhile, is full of politically conservative, Republican-run states that have refused to expand Medicaid, despite the extra federal funding the Affordable Care Act made available for that purpose. Many experts believe expanding Medicaid stabilizes private insurance markets by relieving them of some people with severe medical problems that run up costs for everybody else.

These factors help explain why North Carolina, Oklahoma and Tennessee are among the states that have some of the most troubled insurance markets today.

Conversely, urban counties and those in states that are expanding Medicaid tend to have the healthiest insurance markets. In states like California, Maryland and Michigan, urban and even some rural counties have multiple insurers offering plans with relatively cheap premiums.

Overall, in counties with three or more insurers, 53 percent voted for Clinton, while 41 percent voted for Trump, according to HuffPost’s analysis.

Exactly how these numbers translate politically is, of course, impossible to say.

Trump voters might be reluctant to blame the president for future problems with the insurance markets, even if they are the ones feeling the pain, regardless of what role his administration played in it.

On the other hand, they might blame Trump because he’s the one in charge now. And regardless of how they feel about the president, they might happily focus their wrath on Republican incumbents in the 2018 midterm elections.