These numbers are so important for a general election context because the presidential and congressional outcomes were extremely correlated with each other in 2016 and recent elections. As the scatterplot below demonstrates, relatively few races (those farthest away from the diagonal line) saw a meaningful divergence between the presidential results and the House results.

Trump of course won’t be running for office in 2018, but as the most pivotal figure in American politics for the next four years, he’ll be a singular influence for nearly every voter. Presidential approval has long proven a key factor in determining how the incumbent president’s party fares in downballot elections. That’s particularly true in the last few decades, where partisan polarization has reached highs not seen for many generations. Democrats who hold districts that Trump won might be wary of crossing the voters who elected him, but there are only 12 of those Democrats to begin with.

Polarization doesn’t only mean that Democrats in dark blue districts can be free to follow their hearts and stand fast against everything Trump does. It also means that they could pay a significant penalty if they don’t steadfastly oppose his agenda, which we can be sure is wildly unpopular with Democratic primary voters. However, these primary voters have, in the past, often been far more supportive of compromise and opposed to ideological purity tests than Republicans. That could well change in the era of Trump, though, and it’ll be up to the progressive base to ensure that Democratic incumbents hold the line.

One of the most urgent concerns going through any congressmember’s mind is how political actions will affect their chances of winning re-election. Of course, individual House Democrats also care about gaining an overall majority for their party and are wary of opposing harmful policies that are nonetheless popular. These conflicting concerns can sometimes mean taking actions that put incumbents at risk of losing in a primary or general election.

Unlike Republicans, who nearly all have more to fear from a primary challenge than a general election defeat because Trump won most of their districts by a substantial margin, House Democrats aren’t in the majority. Being in the minority means the few dozen Democrats who hold more marginal seats face a much stronger individual incentive to defect on bad legislation if such measures would pass regardless and are popular with swing voters in their districts.

However, when a commanding majority of House Democrats come from safe districts and Trump is unpopular nationally, those members simply have little to fear from hostile general election voters if they vigorously try to thwart Trump’s agenda. If Democrats can make the GOP own whatever unpopular legislation it ultimately passes without a false veneer of bipartisanship, Team Blue might even reap considerable electoral rewards in 2018, just like Republicans did in 2010.