Tuesday's governor's race in Kentucky showed there is kryptonite to beat President Donald Trump even in rural America, and that kryptonite is health care. Democrats should take that lesson to heart as the 2020 election nears.

Democrat Andy Beshear, who claimed victory in the race on Tuesday, had one unmistakable advantage while running in the overwhelmingly Republican, pro-Trump state: an opponent who had made notable and consistent efforts to take health care from people. Like Trump, Republican Gov. Matt Bevin made cutting health care a top priority for his administration, seeking to end Kentucky's insurance exchange and making the first attempt in the country to add work requirements to Medicaid.

Bevin's efforts went over about as well as Trump's efforts to repeal the Affordable Care Act. As Trump and the GOP learned in 2018, threatening to get in the way of Americans' ability to care for their family members is politically hazardous.

But the biggest question isn't whether Trump learns his lesson from Kentucky; it's whether Democrats do. Kentucky's race presents fresh evidence that Democrats have a winning formula for beating Trump with the right focus on voters' most important issue. They must make it clear that they support access to affordable health care while Republicans are carrying out policies that put it at risk.

So far, the Democratic primary reflects scant understanding of this political advantage. Every Democratic candidate believes Americans should be guaranteed access to affordable health care, in contrast to Trump. Yet rather than locking into their winning position as the protectors of American families, they have become consumed instead with an argument over how Americans should get their coverage. The Sanders-Warren camp doesn't believe private employer-based insurance should be allowed as an option. The Biden-Buttigieg-Klobuchar camp does.

While primaries are perfectly appropriate places to spend some energy flushing out these policy distinctions, Democrats have reason to worry that their party lost the plot by making this its near-singular focus. The question should not be "Are insurance companies popular?" (Hint: They're not.) Rather, it should be "Do Democrats understand what the public cares about?"

There is little consistent evidence that general-election voters care about how they are guaranteed health care protection, only that they are guaranteed protections. It's a matter of political discipline. Beshear has his own credible claim of being a proactive and creative leader on health care, yet he squarely kept the focus on Bevin's record. Likewise, the lion's share of Democrats' 2020 advantage comes from maintaining a disciplined focus on Trump's disastrous record. Democrats seem to know that it's more important to make clear that Trump is a climate-change denier who walked away from the Paris climate agreement than to argue over the fine points of the Green New Deal. The same should be true of Trump's health care record.

In fact, Trump helped the Democrats' argument by joining a lawsuit filed by Texas and other Republican-led states — currently on appeal — which would eliminate the Affordable Care Act root and branch. If the lawsuit succeeds, it will end federal protections for people with pre-existing conditions, throw millions off their insurance and increase drug costs for millions of seniors. Democrats have had surprisingly little to say about it.

Come 2020, the Democratic nominee will have an opportunity to reframe the debate. He or she will face a playing field that looks more like Beshear's in Kentucky than the current primary map. Swing-state voters in Wisconsin, Pennsylvania and North Carolina, who have had a rough go under Trump's health care policies, will be front and center. If given the opportunity, Trump will gladly take the focus off his own dismally unpopular health care record. Getting lost in the details of how universal coverage would work will give him that chance.

A Democratic nominee must consistently paint the picture of how Trump's policies have damaged American families' health and security. Their winning hand is simply that they would sign anything that improves health care coverage for Americans, while Trump has done the opposite.

Both Trump and health care made Kentucky a national race and one of the last reliable signals the next Democratic nominee will get about the general election. Beshear showed that when facing a bully who has targeted Americans' health care, electoral victory is Democrats' for the taking.

Andy Slavitt is former acting administrator of the Centers for Medicare and Medicaid Services in the Obama administration and board chair of United States of Care. He wrote this article for the Washington Post.