To retake the presidency in November, the Democrats will need to win the Electoral College. The clearest path to do that is to win the swing states that had the closest margins in the 2016 election. Fortunately, the Democrats have a playbook to follow from the 2018 midterm election.

There are many factors that led to Democratic victories in the crucial swing states of Michigan, Minnesota and Wisconsin in 2018, and it’s always hard to isolate any single cause. But the successful Democrats all talked about health care — with a focus on fixing the Affordable Care Act and reinforcing Medicare.

I looked at the congressional districts in Michigan, Minnesota and Wisconsin where Democratic House candidates picked up the most votes in the 2018 election relative to the 2016 presidential election. (I excluded Pennsylvania from the analysis because it drew new congressional district lines in 2018.)

Health care was the most prominent message of the Democrats who won those House seats. They focused on creating an option to buy in to Medicare, building upon the Affordable Care Act, and lowering costs of prescription drugs. Those campaign messages seemed to resonate with voters in the upper Midwest states.

Across these three states, 12 Democratic House candidates received more votes than did Hillary Clinton in 2016 in their districts. This feat was impressive because turnout is lower in midterm elections. In these districts, there were between 35,000 and 60,000 fewer votes cast, yet these candidates got between 3,900 and 116,000 more votes than Clinton.

For example, Democrat Ron Kind in Wisconsin’s 3rd District received 187,888 votes in 2018. Hillary Clinton received 160,999 votes in 2016 in that district — even though 48,000 fewer total votes were cast in 2018 than in 2016. Kind and these other candidates were able to capture more votes than Clinton without increasing turnout, which suggests they were able to win the votes of many who had not voted for Clinton in 2016.

The results in the 12 districts I examined — the 1st, 5th, 7th, 8th and 11th in Michigan; the 1st, 2nd, 3rd, 7th and 8th in Minnesota; and the 3rd and 6th in Wisconsin — may be able to offer some insight into the policy platforms that can generate stronger voter support. Ten of these 12 districts cast more votes for Trump than Clinton in 2016 and six were close enough in 2016 that the two presidential candidates were within 10 percentage points of each other. These are not solid Democratic districts.

For my analysis, I examined the campaign webpages from the morning of the election, Nov. 6, 2018. Ten of these 12 websites presented a list of issue priorities, showing how the candidates chose to present themselves to voters seeking information.

For the winners in the 12 districts, health care was a top issue. Eight of the 10 candidates’ websites listed health care first or second in their issue priorities, the remaining two listed health care third. Other important issues listed were supporting veterans, the Flint, Mich., water crisis, education, protecting working families and campaign finance reform. This focus on health care was not limited to these districts. Among all Democratic candidates, health care was a central theme in their 2018 campaign ads.

Among these 10 Democratic candidates, seven proposed allowing those aged 55 to 65 the option to buy in to Medicare, six wrote of fixing and building upon the Affordable Care Act, six supported having a government policy to lower the cost of prescription drugs and four expressed opposition to Republican efforts to repeal the ACA.

None of the candidates’ campaign sites mentioned “Medicare for All.” This suggests Democratic presidential candidates should be cautious about departing from the messages and policy platforms of the 2018 midterm winners, without good reason and careful discussion.

Lynn Vavreck, a political scientist at UCLA, argued in “The Message Matters” that a presidential candidate challenging an incumbent during a strong economy must come up with an “insurgent issue” on which to run the campaign. The insurgent issue should have two characteristics. First, the challenger’s position must be more popular than the president’s. Second, the president must be constrained from changing course and adopting the challenger’s position to co-opt the challenger’s advantage.

With the strength of the economy in 2020, Vavreck’s argument suggests that any Democratic nominee needs an insurgent issue to win. The 2018 outcomes make a good case that health care could be that insurgent issue.

The Democrats’ position is likely to be more popular than President Trump’s, and he is constrained by the Republican effort to repeal the ACA. The health care agendas of Democratic winners in 2018 demonstrate the kinds of policy proposals that work with voters in critical swing states.

Seth Hill is an associate professor of political science at the University of California, San Diego and studies how citizens motivate politician behavior.