PACIFIC JUNCTION, Iowa—Democrats swept to control of the U.S. House of Representatives in 2018. Their ability to keep it in 2020 will hinge in good part on what happens in 21 congressional districts won by both Democratic freshmen in the midterms and by President Trump in 2016.

The challenge for those lawmakers: cultivating a less-partisan approach and working with Republicans, even as many forces pull the Democratic party to the left and press for impeaching the president.

So...